The shower valve is a Moen. In and out regulates flow, and left and right regulates temperature. In order to remove the valve:
Remove the center cap in the knob
unscrew the philips screw in the center, and remove the knob
slide the sleeve off the cartridge cover
unscrew the escutcheon around the valve
remove the copper C clip from the valve
Grip the cartridge with a pair of pliers, and pull it out. You may need to use the tool supplied with the new cartridge to break it free.
Installation is the reverse of removal. If the hot and cold are reversed, simply remove the knob, and rotate the center of the cartridge 180 degrees.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.