The most reliable way is to re-create it:
XDocument doc; // declare outside of the using scope
using (IsolatedStorageFileStream stream = isf.OpenFile("inventories.xml",
FileMode.Open, FileAccess.Read))
{
doc = XDocument.Load(stream);
}
// change the document here
using (IsolatedStorageFileStream stream = isf.OpenFile("inventories.xml",
FileMode.Create, // the most critical mode-flag
FileAccess.Write))
{
doc.Save(stream);
}