read and update xml/xsd file file from pl sql

 sample xsd file

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 <xsd:simpleType name="NODE1">
    <xsd:restriction base="xsd:string">
      <xsd:minLength value="1"/>
      <xsd:maxLength value="50"/>
    </xsd:restriction>
  </xsd:simpleType>
  <xsd:simpleType name="NODE2">
    <xsd:restriction base="xsd:string">
      <xsd:minLength value="1"/>
      <xsd:maxLength value="50"/>
    </xsd:restriction>
  </xsd:simpleType>
  <xsd:simpleType name="NODE3">
    <xsd:restriction base="xsd:string">
      <xsd:minLength value="1"/>
      <xsd:maxLength value="8"/>
    </xsd:restriction>
  </xsd:simpleType>
  <xsd:simpleType name="NODE4">
    <xsd:restriction base="xsd:string">
      <xsd:minLength value="1"/>
      <xsd:maxLength value="80"/>
    </xsd:restriction>
  </xsd:simpleType>
</xsd:schema>


follwing procedure change NODE1 -> maxlength value 50 to 100


create or replace procedure XMLTEST3 is

--Documents variables
doc                                                    DBMS_XMLDOM.DOMDocument;
docelem                                                DBMS_XMLDOM.DOMElement;

--sympleType Nodes
simpleTypeNodeList                                     DBMS_XMLDOM.DOMNodelist;
simpleTypeNode                                         DBMS_XMLDOM.DOMNode;
simpleTypeChildNodeList                                DBMS_XMLDOM.DOMNodelist;
simpleTypeChildNodeMap                                 DBMS_XMLDOM.DOMNamedNodeMap;
simpleTypeChildNodeAttr                                DBMS_XMLDOM.DOMAttr;
simpleTypeNodeListLength                               number;  
simpleTypeAttributeNode                                DBMS_XMLDOM.DOMNode;

--generalTypeCode Nodes
strGeneralTypeCode                                     VARCHAR2(200); 
simpleTypeChildNodeListLength                          number; 
generalTypeCodeAttributeNode                           DBMS_XMLDOM.DOMNode;
strIGeneralTypeCodeAttr                                VARCHAR2(200);
generalTypeCodeMap                                     DBMS_XMLDOM.DOMNamedNodeMap;
generalTypeCodeAttr                                    DBMS_XMLDOM.DOMAttr;
generalTypeCodeNode                                    DBMS_XMLDOM.DOMNode;

--generalTypeChild Nodes
generalTypeCodeChildNodeList                           DBMS_XMLDOM.DOMNodelist;
generalTypeCodeCNListLength                            number;
strGeneralTypeCodeCN                                   VARCHAR2(200);
generalTypeCodeCNMap                                   DBMS_XMLDOM.DOMNamedNodeMap;
generalTypeCodeCNAttr                                  DBMS_XMLDOM.DOMAttr;
generalTypeCodeChildNode                               DBMS_XMLDOM.DOMNode;

--Other Variables
xmlTypeValue                                           XMLType;
--used for write modified xmltype
xmlTypeTOClob                                          CLOB;

--used for write again modified clob to xmltype
clobToXmlType                                          sys.xmlType;

--rowid used to update modified record
row_id                                                 varchar(200);

--file name to check specisfic file
fileName                                               varchar(1000);



BEGIN

 xmlTypeValue := XMLType(
 '
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

 <xsd:simpleType name="NODE1">
    <xsd:restriction base="xsd:string">
      <xsd:minLength value="1"/>
      <xsd:maxLength value="50"/>
    </xsd:restriction>
  </xsd:simpleType>
  <xsd:simpleType name="NODE2">
    <xsd:restriction base="xsd:string">
      <xsd:minLength value="1"/>
      <xsd:maxLength value="50"/>
    </xsd:restriction>
  </xsd:simpleType>
  <xsd:simpleType name="NODE3">
    <xsd:restriction base="xsd:string">
      <xsd:minLength value="1"/>
      <xsd:maxLength value="8"/>
    </xsd:restriction>
  </xsd:simpleType>
  <xsd:simpleType name="NODE4">
    <xsd:restriction base="xsd:string">
      <xsd:minLength value="1"/>
      <xsd:maxLength value="80"/>
    </xsd:restriction>
  </xsd:simpleType>
</xsd:schema>
');



   -- Create DOMDocument handle
  doc     := DBMS_XMLDOM.newDOMDocument(xmlTypeValue);
  DBMS_LOB.createtemporary (xmlTypeTOClob, TRUE);
  DBMS_XMLDOM.WRITETOCLOB(doc, xmlTypeTOClob);

  docelem := DBMS_XMLDOM.getDocumentElement(doc);

  -- get nodelist

  -- Access simpletype elements
  simpleTypeNodeList := DBMS_XMLDOM.getElementsByTagName(docelem, 'simpleType');
  simpleTypeNodeListLength  := dbms_xmldom.getLength(simpleTypeNodeList);
 
  -- loop through simple type elements
  FOR i IN 0 .. simpleTypeNodeListLength - 1 LOOP
        simpleTypeNode := DBMS_XMLDOM.item(simpleTypeNodeList, i);
         -- Get the attributes of this node
        simpleTypeChildNodeMap := DBMS_XMLDOM.GETATTRIBUTES(simpleTypeNode);
       
        -- Find the 'index' attribute
        simpleTypeAttributeNode := DBMS_XMLDOM.GETNAMEDITEM(simpleTypeChildNodeMap,'name');
        simpleTypeChildNodeAttr := DBMS_XMLDOM.MAKEATTR(simpleTypeAttributeNode);
        strGeneralTypeCode := DBMS_XMLDOM.GETVALUE(simpleTypeChildNodeAttr);
       
        IF strGeneralTypeCode = 'NODE2' THEN
       
         simpleTypeChildNodeList := DBMS_XMLDOM.GETCHILDNODES(simpleTypeNode);
         simpleTypeChildNodeListLength := dbms_xmldom.getLength(simpleTypeChildNodeList);
        
         FOR j IN 0 .. simpleTypeChildNodeListLength - 1 LOOP
            -- Get the attributes of this node
            generalTypeCodeNode := DBMS_XMLDOM.item(simpleTypeChildNodeList, j);
            generalTypeCodeMap := DBMS_XMLDOM.GETATTRIBUTES(generalTypeCodeNode);
            generalTypeCodeAttributeNode := DBMS_XMLDOM.GETNAMEDITEM(generalTypeCodeMap,'base');
            generalTypeCodeAttr := DBMS_XMLDOM.MAKEATTR(generalTypeCodeAttributeNode);
            strIGeneralTypeCodeAttr := DBMS_XMLDOM.GETVALUE(generalTypeCodeAttr);
                     
            --final loop
            generalTypeCodeChildNodeList := DBMS_XMLDOM.GETCHILDNODES(generalTypeCodeNode);
            generalTypeCodeCNListLength := dbms_xmldom.getLength(generalTypeCodeChildNodeList);
            
             FOR k IN 0 .. generalTypeCodeCNListLength - 1 LOOP
                -- Get the attributes of this node
                generalTypeCodeChildNode := DBMS_XMLDOM.item(generalTypeCodeChildNodeList, k);
                generalTypeCodeCNMap := DBMS_XMLDOM.GETATTRIBUTES(generalTypeCodeChildNode);
                generalTypeCodeAttributeNode := DBMS_XMLDOM.GETNAMEDITEM(generalTypeCodeCNMap,'value');
                generalTypeCodeCNAttr := DBMS_XMLDOM.MAKEATTR(generalTypeCodeAttributeNode);
                strGeneralTypeCodeCN := DBMS_XMLDOM.GETVALUE(generalTypeCodeCNAttr);                
               
                IF DBMS_XMLDOM.GETNODENAME(generalTypeCodeChildNode) = 'xsd:maxLength' THEN  
                  IF DBMS_XMLDOM.GETVALUE(DBMS_XMLDOM.MAKEATTR(generalTypeCodeAttributeNode)) = '50' THEN
                     DBMS_OUTPUT.PUT_LINE('-------UPDATING RECORD--------BEGIN  ROW ID -'||row_id);
                     DBMS_OUTPUT.PUT_LINE('-------BEFORE CHANGE = '||strGeneralTypeCodeCN);
                     DBMS_XMLDOM.setAttribute(DBMS_XMLDOM.makeElement(generalTypeCodeChildNode), 'value', '100');
                     DBMS_OUTPUT.PUT_LINE('-------AFTER CHANGE = '||DBMS_XMLDOM.GETVALUE(DBMS_XMLDOM.MAKEATTR(generalTypeCodeAttributeNode)));
                     DBMS_XMLDOM.WRITETOCLOB(doc, xmlTypeTOClob);
                     clobToXmlType := sys.xmlType.createXML(xmlTypeTOClob);
                    
                      --update  <table_name> set <column_name> = clobToXmlType where ROWID = row_id;
                      --commit;
                      DBMS_OUTPUT.PUT_LINE('-------UPDATING RECORD--------END  ROW ID -'||row_id);
                      DBMS_OUTPUT.PUT_LINE('');
                   END IF;
                END IF;
               
             END LOOP; --generalTypeCodeCNListLength for
        
        END LOOP; --simpleTypeChildNodeListLength   for
             
        END IF;      
  END LOOP; --simpleTypeNodeListLength for

 
  DBMS_XMLDOM.freeDocument(doc);




end XMLTEST3;


You can do this from updateXML and xpath also 

send me and email (saneera@gmail.com) if there is any question regarding this

Thanks