How do I replace a tub faucet seat in which the hex head is stripped out?

Buy a square seat wrench that is tapered, make sure it will tighten into the seat without hitting the back of the faucet. Tap in a few times with a small hammer and then use an adjustable wrench to try to turn the seat wrench, if at first you don't succeed try again. Do not drive the wrench in to far or it will swell the threads into the threads in the faucet body and then will not turn at all.