Turn the water supply to the shower off by closing the home's main water line at the water heater as a precaution.
Remove the cover plate screws on either side of the faucet handle. These screws are usually located at the 3 o'clock and 9 o'clock positions, one on each side. On some models, the screws are located below the handle. Turn the screws counterclockwise with a screwdriver to remove them.
Pull out the screws and pull the plate off of the shower wall with your hands. If there is a bracket underneath the plate, remove the screws that hold the bracket to the wall by turning them counterclockwise then pull the bracket off.
Place the new trim plate onto the shower wall over the faucet. Insert the mounting screws into the screw holes, thread them into the holes then tighten them in a clockwise direction. If the trim plate requires a bracket, mount this to the shower valve first by setting it on the shower valve and screwing in the mounting screws. Then place the cover plate onto the bracket and screw in the cover plate mounting screws by turning them clockwise.