You really need a press to get a proper fit. Heating it with a heat gun until it starts to give, and trying to get a good fit, will be very challenging. That can work for adjusting an improperly fitted holster or sheath though, but even then, you need ot be careful.
Most commercial holsters are made in a mold, and there is more to worry about besides the gun pocket itself, especially if you get carried away with the heat.
Worst that can happen is you have to buy a new holster.
Im betting thats going to be the result.
Theres a bit more to it, than just heating it up and jamming the gun in there.
Its one thing to start from scratch, and have the tools. Trying to take a pre made holster, and have it "properly" fit something else, especially without tools, is going to be difficult. Kydex isnt hard to work with, and its really a lot of fun, but things can get complex in a hurry, and require a good bit of thought, and some skill, if you want a good, functional job.