A series of splines or small teethlike gears on the back of the shower knob integrate with another series on the faucet's valve stem. When you turn the knob, its splines contact those on the valve stem, increasing or decreasing the flow of water through the faucet. If the shower knob spins uselessly, splines are corroded or stripped on the knob, the valve stem, or both.
Shut off the water in your house by turning the main water valve to the closed position. Turn on a faucet located on a lower level than the shower's location, releasing any leftover water pressure in the pipes.
Locate and remove the set screw that holds the knob onto the faucet assembly. On some faucets the screw is located under a domed cap on the knob, while on other faucets the screw sits in a small hole on the side of the knob.
Pull the knob toward you to remove it from the faucet. Clamp and handle puller tool to the knob if you cannot remove it by hand, then turn the tool's post clockwise until the knob comes free.
Examine the splines on the inside of the shower knob, looking for damage or corrosion. Look over the splines on the end of the shower valve stem for damage or corrosion as well. If the valve stem needs to be replaced, turn it counterclockwise with an adjustable wrench until it comes out of the pipe.
Put the faucet together with the new valve stem or knob by reversing the steps to disassemble the faucet. Restore the water in your house by slowly turning the main valve to the open position.