I find solution:
private void liveClient_DownloadCompleted(object sender, LiveDownloadCompletedEventArgs e)
{
Stream stream = e.Result;
using (IsolatedStorageFile storage = IsolatedStorageFile.GetUserStoreForApplication())
{
using (IsolatedStorageFileStream fileToSave = storage.OpenFile("tasks.xml", FileMode.Create, FileAccess.ReadWrite))
{
stream.CopyTo(fileToSave);
stream.Flush();
stream.Close();
}
}
}