Having several non-stick frying pans that could double as bowls when upside down, and little to lose, I tried an experiment.
Taking a steel 1" plate, a small hammer (1" flat face) a thich towel, and the pans.
I took the towel, placed over the pan which was laid on top of the steel. Now, raising the hammer approximately 12", I made around 20 strikes going from the center outwards in a spiral pattern.
I then looked to see if I'd made any difference in the approximately
1/4" bow (over 10") diameter.
I was rather surprised to find that the pan was now almost entirely flat, with very little curve. Repeated this on a smaller pan, with good results.
A firm surface that's harder than the frying pan is a must, I happened to have some steel plate lying around. A bit of granite plate, or even a flat concrete floor should do just as well.