I've pull UTHs off of two different tanks. The exo terra one I had ripped because it was the think type of UTH thats like heat tape the other was more successful which was a Zoo Med Repti whatever. With the Zoo Med one I took an old Visa Gift card and slowly started to break the bond between the tank and UTH while gently pulling it. and it came off just fine. I just had to use Duct Tape to put on another tank
I used GOo be Gone on one that didn't want to come unstuck. That stuff is AWESOME. So if it doesn't seem to want to peel up, you can get the edge up a little, spray the Goo Be Gone, and rub it into the crevice, and it'll just start turning loose. Just keep poking it into the space between the UTH and the glass, it'll keep peeling.
The other times I've just peeled, and it's been fine too. I did stick one to the side of my head, when it popped free unexpectedly and I sort of flung it towards me. Don't do that, it sucks.