Q:

XDocument.Save(string) not available

I'm using VS 2012 Express for Windows 8.I want to load an XML file, modify its content and then save it back to disk.
Up to now, I've been using LINQ to XML and I've been able to load the file, change some node information. 
I want to save the file back to the disk using the XDocument.Save(string) method but the intellisense does not include that method though it is documented in the online documentation. 
Any idea why?
Thanks
---UPDATE---
Here's what I'm trying to do
string questionsXMLPath;
XDocument xmlDocQuestions = null;
StorageFile file = null;

public MainPage()
{
    this.InitializeComponent();

    questionsXMLPath = Path.Combine(Package.Current.InstalledLocation.Path, "Assets/Template.xml");
    xmlDocQuestions = XDocument.Load(questionsXMLPath);
}

private async void SomeCodeHereToPopulateControls()
{
    // This Code populates the controls on the Window to edit the XML nodes.
}

private async void Button_Click_3(object sender, RoutedEventArgs e)
{
    XElement eleQuestion =
        (from el in xmlDocQuestions.Descendants("Question")
        where (string)el.Element("ID") == txtID.Text
        select el).FirstOrDefault();

    eleQuestion.Elements("Description").FirstOrDefault().ReplaceWith(txtDescription.Text);

    xmlDocQuestions.Save(questionsXMLPath);  // ERROR HERE AND CAN'T COMPILE
}

A:

You'll need to use the Windows.Storage APIs. In the sandboxed and asynchronous world of Windows 8, you'll find two significant differences in dealing with file storage:

The application can programmatically access data only from the "local storage" for that application UNLESS the end-user has granted specific permission to the app to store/read from elsewhere in the file system (via the file and folder pickers)
Reading and writing to files is an asynchronous operation, so you'll find most of the file access methods ending in "Async" and you'll use (typically) the async/await pattern to leverage them

Take a look at the File access and permissions in Windows Store apps topic on the Windows Dev Center for more details as well as the File access sample.  
In your specific case, you'll just end up writing XDocument.ToString() to the desired output file using the techniques shown in the article and the code sample cited above.
By the way, for a more holistic and measured approach to learning about the file system (as well as other concepts unique to Windows Store programming), the (free) Microsoft App Builder program is a great way to spin up.

