I have heard that freeze spray works on newer machines but have never tried it myself
DO NOT use it on older machines as the guys suggest leave the of pfield in the hot sun or use a heat gun.
If you use a heat gun make sure you don't overheat or you will soften the paint and it will lift off with the mylar.
I used the heat gun method on a pfield that had Mylar put on after planking and paint loss had already occurred and I lost almost no paint at all.
White Spirits works great for glue removal it's nice and thin an gets the glue out of all the cracks.
It's also very cheap and available from any supermarket

If the Mylar is not lifting and is still in good condition then there is no real need to remove it
If you had small areas lifting at corners or something you could just trim them off with a sharp knife
Run around the edges with a rag and white spirits to clean up the dirty mark and you will be surprised how much better it looks, without the risk or stress of losing paint