XML : Writing data to existing Xml

I am developing a universal windows app on windows 10 with Visual Studio 2015 and have a pretty large Xml structured like this:

  <header id = "1">      <title>          some text      </title>      <question>          a question      </question>      <user_input>          <input1>          </input1>          <input2>          </input2>      </user_input>  </header>    <header id = "2">      <title>          some text      </title>      <question>          a question      </question>      <user_input>          <input1>          </input1>          <input2>          </input2>      </user_input>  </header>  ...    

This is repeating many times. There are parts that should never be changed (e.g. title, question). Now i want to write new elements into "ui", so it can be read again and shows the new content in texbox. I use a FileStream and XmlDocument and XmlNodeList to read the Xml and show the content on textblocks:

  path = "test.xml";  FileStream stream = new Filestream(path, FileMode.Open, FileAcces.Read);  XmlDocument xdoc = new XmlDocument();  xdoc.Load(reader);  XmlNodeList node = xdoc.GetElementsByTagName("header");  textblock1.Text = node[0].Attributes["id"].Value;  textblock2.Text = node[i].ChildNode[1].InnerText;  ....    

I tried this to write into the Xml:

  XDocument xdoc = XDocument.Load(path);  XElement ele = xdoc.Element("header");  ele.Add(new XElement("user_input",          new XElement("input1", newtext)));          xdoc.Save(path); <---- at this point there is an error    

"Argument 1: cannot convert from 'string' to 'System.IO.Stream'"

My question is: how can i write the user input (some string) to the place I want it to be? The first input shall be written into header with id = 1 into user_input, the second into header id = "2" and so on. I already tried to load the xml with XDocument and write a new element with XElement, but it work at all.Is there something wrong with my xml? Or is it the function? Thank you in advance.