XML : how to edit xml file in windows store app

I am making an app in windows store and having a problem in writing xml in created xml file. I have followed this Editing XML file in Windows Store App but it didn't work for me.

I want this xml to be wrote in my xml file on button click.

  <drink>      <drinkImage>ck.png</drinkImage>      <drinkTitle>COKE</drinkTitle>      <drinkDescription>(1793-1844)</drinkDescription>    </drink>    

my current file is this:

     <?xml version="1.0" encoding="utf-8" ?>      <drinks>        <drink>          <drinkImage>pepsi.png</drinkImage>          <drinkTitle>PEPSI</drinkTitle>          <drinkDescription>(1793-1844)</drinkDescription>        </drink>  **<here I Want above xml on button click>**      </drinks>    

Here is what I have tried:

  namespace DrinksApp  {      /// <summary>      /// An empty page that can be used on its own or navigated to within a Frame.      /// </summary>      public sealed partial class coke : Page      {          public coke()          {              this.InitializeComponent();            }          XmlDocument dom = new XmlDocument();              private void Button_Click(object sender, RoutedEventArgs e)          {              this.Frame.Navigate(typeof(softdrinks));          }            private async void Button_Click_1(object sender, RoutedEventArgs e)          {              StorageFolder sf = await ApplicationData.Current.LocalFolder.CreateFolderAsync("favourite", CreationCollisionOption.OpenIfExists);              StorageFile st = await sf.CreateFileAsync("fav.xml", CreationCollisionOption.OpenIfExists);              var content = await FileIO.ReadTextAsync(st);                if (!string.IsNullOrEmpty(content))              {                  dom.LoadXml(content);                  var existingRoot = dom.SelectSingleNode("//drink");                  var childTag = dom.CreateElement("drink");                  childTag.InnerText = "ck.png";                  existingRoot.AppendChild(childTag);                  var childertag = dom.CreateElement("drinkImage");                  childertag.InnerText = "COKE";                  childTag.AppendChild(childertag);                  var childertag2 = dom.CreateElement("drinkTitle");                  childertag2.InnerText = "Nothing";                  childTag.AppendChild(childertag2);                  var childertag3 = dom.CreateElement("drinkDescription");                  childTag.AppendChild(childertag3);                    await dom.SaveToFileAsync(st);              }                }              }  }    

I already have an xml file as mentioned above: