You can only slowly tear it off. There's no other good ways. Thinners or similar things will damage the glass.
You can wipe it off using paper with some alcohol (Industrial alcohol is better while medical alcohol is also acceptable.). Then scratch it. The film will be cleaned up.
Besides, you can use nail polish remover. The way of use is the same as that of alcohol and acetone. The effect is quite good. The quality of the polish remover can be either good or ordinary as long as it can remove the nail polish.
