Overwrite / Replace XML data by xml string source

I dont have much experience with XML.

My XML looks like this

<IniFile xmlns="http://IniFile">
  <IniLine>
    <IniKey>Firma</IniKey>
    <IniValue>Test1</IniValue>
  </IniLine
</IniFile>

I already got my reading Methods.

I need to override Firma and Test1. My XML source is a string in my Database. I can add some new elements and new data but I cant change the values in it. Is it even possible?