Ranges are kind of a tricky concept. But make sure you're showing your bookmarks, which will help you understand whether your bookmark "TestBM" looks like this:
Isome text
or like this
[some text]

On one case-- the bookmark is just like an insertion point before "some text" ... in the other, it is bracketing "some text." If it is already bracketing text, then Greg's code is perfect. If it isn't already bracketing text... then your question doesn't really make sense. There's nothing to "overwrite" if your bookmark is just an insertion point.