You can peel it off and sand it and re-cover it. If it is old, I would do that. RE-covering over top won't work. If you try to just trim it and it is really old, it gets brittle and the heat for the new will blow holes in it.

If you have trouble getting some off, heat it. You will leave some residue on the plane, but you can sand that off pretty easy and get it back to balsa quick. Don't over sand it. Re-covering is sometimes the best option.