Use a sharp Stanley knife/boxcutter and a metal anti-slip ruler, on top of the (window) film to protect the film from cutting inward into the (window) film itself. Press firmly on the ruler. Preferably a 2 men/women job. One cutting, one holding the ruler. A safety ruler, with a anti-slip back, on top of the film works best.
Permanently removing (window) film
Use a heat gun (or something similar, the sun, portable infrared heater) to heat the vinyl itself during removal and you should be able to pull the vinyl and the adhesive from the mounting surface (glass/window) in one go. If the adhesive from the film remains on the glass (doesn’t stick to the vinyl itself) you likely have to heat the vinyl to a higher temperature. More info you will find here.