A "shower diverter" is a plumbing technical term for the valve you turn or switch to change the flow of water from a shower head to the bathtub faucet or spigot. These devices are installed in combination shower/tub fixtures, allowing the homeowner to either bathe or shower at her preference. These are typically found in guest or secondary bathrooms and are wonderful for accommodating both adults and children. If your shower diverter will not stay on, you must replace it.
Put a towel over the drain hole to prevent parts from falling into the drain.
Turn off the water supply to the bathroom.
Remove the knob from the diverter by unfastening the retaining screw in the center of the handle, exposing the stem and diverter.
Clamp a wrench around the diverter and turn it counterclockwise until it comes free from the water-supply line recessed inside the wall.
Wrap a thin layer of thread tape around the threads on the new diverter.
Fit the new diverter onto the water-supply line and tighten it clockwise with a wrench.
Put the handle on the new diverter and fasten the retaining screw with a screwdriver to finish the shower diverter replacement.
Remove the towel from over the drain and turn the water supply back on.