Replace the diverter/tub spout if it is leaking by. Go to a plumbing shop and buy the same brand you have if you have a name brand metal faucet. The replacement diverters/tub spouts sold at some discount stores are plastic and don't work as well and may require a different nipple.
They just unscrew but some brands have a set screw underneath that needs to be loosened with an allen wrench first. Shut off water. Pull away from wall while unscrewing to avoid wall damage. There is a 1/2" nipple stub out that will either unscrew from the diverter or out of the wall. Use teflon tape when replacing. Different brands of diverters/tub spouts have the threaded part at different depths. Using an offbreed brand may require that you replace and/or cut&glue a new nipple.