Let the car sit in the sun on a warm day or use a blow dryer on high and hot setting. I wouldn't use a heat gun unless you've done it before.
Starting on one corner lift the edge back and pull slightly as you go at about a 45 degree angle, not straight up. Don't rush. This is a patience thing. Go slowly. If you are using a dryer keep it about 10-12 inches away, constantly moving and peel back small sections at a time.
There will most likely be adhesive residue left that you will need to clean off. Detailer’s clay and /or 1:1 Isopropyl Alcohol / distilled water will clean any adhesive residue.