The first thing you need to do is to spread a plastic sheet on the car seats and the floor to avoid any damage to the upholstery while you remove the tint.
Spray a little water around the outside boundaries of the tint using a spray bottle. Use just the right amount of water. Remember that you just need to spray the tint, not wash it.
Cut a few plastic bags in the shape of the tinted window. Use these plastic bags to cover the tinted window glass. The plastic bags will absorb a lot of heat quickly, thus helping to warm up the window glass in less time. When the glass warms up, it loosens the adhesive sticking the tint to the glass window.
Now empty the spray bottle and fill it with ammonia this time. Spray ammonia freely on the inside of the tinted window. Spray a fairly large quantity of ammonia and try to do it as quickly as possible. Then, with another plastic bag cut in the shape of the car window, cover the area.
You need to give time to the ammonia to soak into the window tint. 90 minutes would suffice. When you come back after 90 minutes, you are all set to go to remove the tint.
Starting at the corner of the window, begin peeling away the tint using a razor blade. Ideally, the tint should peel away in a single large piece. If the tint breaks at some point, you need to restart peeling it at the edge. Be careful while using the razor blade. You do not want to damage the window glass with the razor blade.
When you are done removing the tint, there may be some adhesive material left on the window glass. Apply a quality adhesive remover such as Quickie Professional Gunk or Goo Remover on a steel sponge. Scrub the glass window using with the steel sponge to get rid of the adhesive material.
Finally, using a glass cleaning liquid and a clean towel, wipe the car window to properly clean it.