THis is code for updating
Qt Code: 
  1. QFile inFile("C:\\test.xml" );
  2.       inFile.open( QIODevice::ReadWrite | QIODevice::Text );
  3.       QDomDocument document;
  4.       document.setContent( &inFile ) ;
  5.  
  6.  
  7.       QDomElement documentElement = document.documentElement();
  8.         QDomNodeList elements = documentElement.elementsByTagName( "TextWatermark" );
  9.         QDomElement bar = elements.at(0).toElement();
  10.  
  11.         if(bar.hasAttribute("FontName"))
  12.             bar.setAttribute("FontName","Thanh Hung");
  13.  
  14.         QFile outFile( "C:\\test.xml" );
  15.           if( !outFile.open( QIODevice::WriteOnly | QIODevice::Text ) )
  16.           {
  17.             qDebug( "Failed to open file for writing." );
  18.             //return 0;
  19.           }
  20.  
  21.           QTextStream stream( &outFile );
  22.           stream << document.toString();
  23.  
  24.           outFile.close();
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