In the center of each handle is a little button that either prys off with a knife, or possibly unscrews. Then you will see a screw which holds the handle on. You might need a puller tool to actually pull the handle off.
The pyramid-shaped flanges will unscrew. Be very careful not to damage these as replacements may be diffficult to find.
Once the flange is off, turn off your water supply, and unscrew the entire stem from the body in the wall. Partially open each stem before trying to unscrew it. Take the stems to a good supply house for replacement, or at least new rubber parts. Also, get a stem wrench to unscrew the stem which is inside the body. It is the piece that the washer on the end of the stem contacts when you turn the faucet off. At the store, they should recognize the brand of stem and can give you the proper seat.