So long as you can pull the window apart it should be no big deal. The pane should be held in place by something like glazing clips on the inside. Pull those and the glass should come out. You may have to remove some glazing putty on the outside too. Take the pane to a glass shop and have either a single pane or a double one cut for you if that is what it turns out to be. Glaze it in place and reassemble the frame.