Work the tip of a plastic scraper into a corner of the film to try to release some of the film or manually peel as much of the film off as you can. This will help the acetone work its way between the glass and the film.
Put on protective gloves and pour some acetone onto a clean rag. The rag should be well coated but not dripping. Acetone dries quickly so you will probably have to reapply it throughout the process.
Hold the acetone against the film and begin rubbing in a circular motion. Pull the film off as it begins to release. If it comes off in pieces, continue to apply the acetone and pull at the film until all of it is released.