How to update xml node value?

I use function mdlXMLDomElement_getValue get xml node value ok.

But use mdlXMLDomElement_setValue update xml node value fail.

How to update xml node value?such as update 85 to 100.

xml define:

<rootNode>

<ChildNode>

               <value Var="test">85</value>
         </ChildNode>
</rootNode>

My code:

if(SUCCESS==mdlXMLDomNode_selectSingleNode (&symbNodeRef,rootNodeRef,pNodeName))
{

if(SUCCESS == mdlXMLDomNodeList_getNextChildElement(&ChildNodeRef, NULL, &nodeListRef, symbNodeRef))

{
         MSWChar pKeyString[256], pValueString[256]; 
         int KeyMaxLen = 256,ValueMaxLen = 256; 
         char keyString[256];

         mdlXMLDomElement_getAttribute(pKeyString, &KeyMaxLen, ChildNodeRef, L"Var", XMLDATATYPE_WIDESTRING); 
         mdlCnv_convertUnicodeToMultibyte(pKeyString, -1, keyString, 1024);
        if (0 == strcmp(pInKey, keyString))
       {
            int iTest=100;  

 char buf[20];
 sprintf_s(buf,"%d",iTest);

            mdlCnv_convertMultibyteToUnicode(buf, -1, pValueString, ValueMaxLen);
            mdlXMLDomElement_setValue (ChildNodeRef, pValueString);

      }

}

}

Thanks!