Replace an existing xml tag value in qt

I want to replace a value of a xml tag. I can replace the value by using QDom, however I am getting a change in my xml file just after that change.

I am working with qt and I have tried with "QDOM" to replace the tag value and save it using "QTextStream".

QFile myFile(QLatin1String(my_FILE));
if (liceFile.open(QIODevice::ReadWrite)) {
    QDomDocument doc;
    doc.setContent(&myFile);
    QDomElement parentEle = doc.firstChildElement("Root");
    QDomNode myValue= parentEle.firstChildElement("Mytag");
    myValue.firstChild().setNodeValue("Hey");
    QTextStream stream(&myFile);
    doc.save(stream, 4);
    myFile.close();



The xml file
"<Root>
   <Mytag>Hello</Mytag>
   <User>Mr</User>
   <Pass>121</Pass>"
If I replacing  the value of "Mytag". 
Output after replace.

"<Root>
  <Mytag>Hey</Mytag>
  <User/>
  <Pass>121</Pass>" 
 I can not understand why <user> tag changed from "<User>Mr<User/>" to   "<User/>"