In bathtub and shower combinations, the diverter valve is incorporated into the bathtub spout. To direct the flow of water to the showerhead, you must pull up on the diverter's handle. A broken diverter will either leak water when the shower is supposed to be turned on, or it will not direct any water to the showerhead. Once the diverter has worn out, you will need to install a new spout with the diverter valve in the same portion of the spout.
Use a flashlight to check the bottom side of the bathtub spout for a screw holding the spout in place. Use an Allen wrench to remove the screw, if one is present, and pull the spout straight off the water pipe.
Wrap a hand towel around the water spout if you do not see a screw on the bottom. Clamp a pipe wrench onto the spout over the towel, then turn the spout counterclockwise until it comes off the water pipe.
Install the replacement spout in the reverse order as you removed the original spout. If the spout does not sit tightly on the water pipe, remove it and screw the plastic adaptor sleeve into the back of the spout, then install it on the pipe again.