Well, I don't know how old your tub is, but you pull the knob and trim ring off. You may have a white ring with two screws holding it down. That is a safety device. Take it off and set it aside. If not, skip to -
Then pull the snap ring out carefully. Note how it was inserted - there may be a right and wrong way. The newer tubs redesigned it so it doesn't matter, the older tubs it very much matters.
Then put the knob - no trim ring - on the valve and turn it as you pull it up and out. If it is stuck, you can try to turn on the pump and let the water pressure help you, but that is a real mess, hard to control, and the core can come flying out and hit you. So keep it as a very last resort.
8-)