Removing window film can be a very easy process, but it all depends on how long the film has been on the glass, and the type of film that was used.

Here's a video tutorial which shows the basic principal.

Typically, films are either made using PVC or PET.

PVC is  a soft material which can be peeled off the glass with a glass scraper. Any glue residue can be removed with soapy water and a scraper.

PET is a brittle material widely used for solar and safety film, and might be made of layers of mettalised material. To remove this type of film, use a wide glass scraper and lubricate with soapy water to remove the film.

As a rule, old PET film are more difficult to remove than PVC films.

For large panes, it is best to first score the film into strips with a sharp knife. This means that you can tackle it in manageable pieces.