Peel the insulation film away from the adhesive or tape. If the film does not peel away easily, use a hairdryer to apply heat to that section. The heat should loosen the glue and allow you to peel the film away.
Use the hairdryer to apply heat to one section of the adhesive. Scrape the adhesive away with a plastic card or a plastic putty knife. Apply heat as needed to continue scraping until the adhesive is completely removed.
Apply rubbing alcohol to a soft cloth and wipe any remaining adhesive from the window. Use gentle pressure to scrub the adhesive away. Depending on how much adhesive is left on the window, you may need to reapply the rubbing alcohol several times.