title: How do I update an existing element of an ObservableCollection?
body: <p>I have an instance of ObservableCollection bound to a WPF listbox with two separate data templates (one for display, one for editing).  The data template for editing has a one-way binding on the textbox, and a Save button.</p>
  
  <p>What changes do I need to make so that when I press the Save button (after putting the list item in edit mode), the value I change the textbox to replaces the value in the ObservableCollection (and the display)?</p>