Is it regular window film or contact paper? With either you need to find a corner where you can start pulling (even if you have to cut one) and pull gently until it lifts away. You can warm it with a hairdryer to loosen the adhesive. Then, scrape it with a razor scraper . Any residue can be removed with the razor and dabs of Goo Gone.
If it is really stuck on, try aiming a hair dryer on the film on a low setting until the film softens enough to pull up. You can help lift the film by running a plastic credit card underneath the film to help lift it.