Shut off the water to the shower by turning the water shut off valve clockwise until it will no longer turn. The shut off valve is a small silver knob that is located either on the wall near the shower, or in a hidden utility compartment on a nearby wall. It can also be located in the basement or in the crawl space of the building.
Locate the screw in the diverter valve and remove it using a screwdriver. If the shower does not have a separate diverter valve, then the diverter is located inside the faucet itself. Remove the screws in the faucet which are usually located in the underside. If the faucet does not have a screw, then it simply turns it counterclockwise to remove it.
Place a wrench around the shower diverter valve which should now be clearly sticking out of the wall. Turn the valve counterclockwise to remove it from the wall. Some diverter valves have a nut located on the exterior that must be loosened with the wrench before the entire valve comes loose.
Pull the diverter valve out of the wall and remove the black rubber washer located on the end.
Take the rubber washer to a hardware store and purchase one that is the exact same size.
Insert the new washer onto the end of the diverter valve that faces the wall. Insert the valve back into the wall and twist it clockwise to reinstall it.
Reinstall the faucet or diverter by reversing the steps taken to remove it.
Turn the water shut off valve counterclockwise to open the flow of water. Turn the water on in the shower and test the shower head to see if the water flow is strong.