XML : Change values in xml, Xamarin ios C#

I'm want change values in xml, but it isn't changing

My code:

  var doc = new XmlDocument();          var documentsPath = System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal);          var filePath = Path.Combine(documentsPath, "myFile.xml");          doc.Load (filePath);          Console.WriteLine ("Zagruzilsa");          Console.WriteLine (doc.OuterXml);        send_zakaz.TouchUpInside += (o,s) => {                using (var client = new WebClient ()) {                street = ulitsa.Text;              kvart = kvartira.Text;              house = dom.Text;              FIO = fio.Text;              code = kod.Text;              telephone = tel.Text;              comment = coment.Text;                var values = new NameValueCollection ();                values ["Order"] = doc.OuterXml;              values ["Order"] = values ["Order"].Replace ("{StreetName}", street.ToString());               values ["Order"] = values ["Order"].Replace ("{FlatName}", kvart);              values ["Order"] = values ["Order"].Replace ("{HouseName}", house);              values ["Order"] = values ["Order"].Replace ("{FIO}", FIO);              values ["Order"] = values ["Order"].Replace ("{Code}", code);              values ["Order"] = values ["Order"].Replace ("{Phone}", telephone);              values ["Order"] = values ["Order"].Replace ("..", comment);              Console.WriteLine("Proverka "+street);              Console.WriteLine(doc.OuterXml);                  values ["OrderText"] = "hello";    

I have same xml like in start of activity. What is wrong in my code? Thanks for help.