Spent 11 years in upholstery.
I wouldn't heat it up, because that will make it expand.
What you need is an upholstery regulator to feed it back in. Putting your left hand at the crease where it meets the stitching (opposite if your left handed), push the slack up, place the regulator in the center of the area that has come loose and feed it back under.
I wish I was closer to you, I could do this is 5 minutes or less.
This is an upholstery regulator.
https://www.ebay.com/itm/141615087312
The tip on this tool is not sharp, it is rounded and wont damage the leather
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Roto-Fab intake, SW 1 7/8" LT w/hi flow cats, Flowmaster outlaw exhaust, UDP, 160* stat, Circle D 3200, Moroso catch can, TCI line lock, Zex Nitrous 125 shot, 3.91 gear, Eaton True-trac, DSS 1000hp axles.