A shower faucet's valve stems fit inside the faucet's handles, where splines or tiny teeth on both the valve stems and the inside of the handles fit together. Each faucet handle has one valve stem that fits inside it. If the valve stems' splines strip out, then turning the handle has limited or no effect on the valve stems, which in turn control the flow of water through the faucet. You must replace the old valve stems with exact matches, restoring the faucet back to its normal operating condition.
Shut off the water supply lines to the shower by closing the main water valve for the house. If possible, turn on another faucet at a lower level of the house away from the shower's faucet, releasing any water left in the supply pipes.
Remove the screws from the middle or sides of the shower faucet handles, using either an Allen wrench or a Phillips screwdriver. Slide the handles off the faucet, and then use an adjustable wrench to turn the retaining nut counterclockwise off the valve stems.
Pull off the metal sleeve, if one is present, from over the valve stems. Clamp the adjustable wrench to the hexagon-shaped bottoms of the valve stems and turn them counterclockwise to remove them. If the wrench does not work, insert a bath socket wrench over the valve stems and use it to turn the stems.
Turn the valve seats out of the pipes with a seat wrench. Coat the new seats with pipe dope and then tighten them into place with the seat wrench. Coat the new valve stems with plumber's grease and tighten them in the faucet with the adjustable wrench or bath socket wrench.
Assemble the rest of the faucet by reversing the way you took it apart. Restore the water to the faucet by opening the house's main water valve. Turn on the shower faucet to check for leaks.