If I understand correctly, you edit an item which is not the selected one, and you need to change the header of the selected one in the Edited event handler. If this is true, this seems to me user unfriendly - it is somehow unexpected to edit an item which is not selected.
As for the concrete question - you can set the header of the SelectedItem if it returns RadTreeViewItem, if it returns a DataItem (the ViewModel to which the RadTreeViewItem is bound) set the property to which the Header of the RadTreeViewItem is bound.
If I understand your scenario incorrectly, is it possible for you to send us more detailed explanation of your use case in steps ? This way we would be better able to advice you.
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