How to uninstall shower knobs
remove the control handle then unscrew the octagonal cover this should reveal the valve top.
take photos and measurements. go to the hardware store and buy as washer kit to rebuild the valve
Now turn ff the water supply (or supplies)
turn the valves on and make sure that the is not a fast stream of water.
now remove the top part of the valves (it unscrews)
screw the control stem into top part of the valve stem in until it come out through the back and replace the O rings.
this is a good time to also replace the washer that controls the water.
lubricate and reassemble using a new fibre washer where the valve body closes.