Save xml file in windows phone 7

How can I save xml file (code) ?
Hello everybody. I can read the xml file, but I can't save xml file :
Sample :
  public class Subject
  {
  public int Id
  {
  get;
  set;
  }
  public Subject()
  {}
  public string Name
  {
  get;
  set;
  }
  public string Number
  {
  get;
  set;
  }
  }
  public class Student
  {
  public string Id
  {
  get;
  set;
  }
  public Student()
  {}
  public string Name
  {
  get;
  set;
  }
 
  public string IconURI
  {
  get;
  set;
  }
  public List Subjects
  {
  get;
  set;
  }
  }
  public partial class MainPage : PhoneApplicationPage
  {
  public MainPage()
  {
  InitializeComponent();
  }
  public List Students
  {
  get;
  set;
  }
  }
  private void LoadXML()
  {
  Uri uri = new Uri("/DemoWP7;component/Save.xml", UriKind.Relative);
  StreamResourceInfo xml = App.GetResourceStream(uri);
  XDocument doc = XDocument.Load(xml.Stream);
  XElement menu = doc.Descendants("menu").First();
  Students = (from student in menu.Descendants("student")
  select new Student()
  {
  Id = student.Attribute("id").Value,
  Name = student.Attribute("name").Value,
  IconURI = student.Attribute("icon").Value,
  Subjects = (from subject in student.Descendants("subject")
  select new Subject()
  {
  Name = subject.Attribute("name").Value,
  Number = subject.Attribute("number").Value,
  }).ToList()
  }).ToList();
  }
I need save the file .xml same this, after that LoadXML()
 
 
 
    number="72">
 
    number="85">
 
 
 
    number="69">
 
 
 
How can I save xml file (code) ?