I heard extended fasting can tighten skin, but it will fuck you up in other ways, so don't do that. Drink lots of water, eat some vitamins that will promote collagen production like vitamin C and omega 3. Stay away from stuff that will dehydrate the skin, like smoking and booze.
I heard some anecdotal stuff about dermatologists using some kind of EMF wand to stimulate skin regeneration, but it could be bullshit quackery sold to old ladies with turkey neck. Same goes for cryonic treatments, it might work, could be bullshit. Otherwise, it should recover over time, depending on your age and genetics. Some women can come back after having a kid no problem, others carry stretch marks for a while.