Editing a xml file located project root on WP8

I want to change a value of an XML file which is located in project root folder.

Following code can read "isFirstUse" and changes the value "YES" to "NO".

I have a XML file named "Values.xml" in project root, however, I can't save changes on file.

public void writeFile()
    {
        XDocument xDoc = XDocument.Load("Values.xml");
        lblSafakCount.Text = xDoc.Descendants("isFirstUse").FirstOrDefault().Value.ToString();

        XElement xElm = xDoc.Descendants("isFirstUse").FirstOrDefault();
        xElm.Value = "NO";
        xElm.Save("Values.xml");
    }

"xElm.Save("Values.xml")" code not saving, it requires stream or textwriter and denies string. How can I edit local(project root, not phone storage) XML file? Thanks.