reheat it with the tourch. not real hot just enough so that when you hit it with a really wet water rag it will shrink. do about as many times as needed untill it's back in shape.
I am not sure if this can be over done. I have done it in the past but quite when I though I was close.
I also have cut it with a cutting wheel to reelieve the excess and then when finished wire welded the damage back up and did the necessary body work, filling & sanding.
