generally you can lift a corner up off the glass with a razor, then peel the plastic off. Go back with Prep-Sol or some other adhesive solvent to get the glue off. If it wont peel, get a razor and make sure you use tons of glass cleaner for lube, make sure the blade doesnt get dull, make sure you have the right side of the blade up if its beveled, and dont slide it backwards on the glass.