Are you sure that's mylar? I dont think mylar goes brittle like that, it could be some kind of plastic tape. I use naptha in a syringe and a plastic razor blade. Works great on old (pre-diamond plate) playfields. Squirt a little naptha on the edge to loosen the mylar, gently work the edge up with the plastic razor blade. When you have enough of a lifted edge, you can start slowly pulling the mylar back over itself.