Turn the water supply shutoff valve leading to your bathtub clockwise to prevent water leaks during the installation. Turn on the shower to remove all of the water from the pipes.
Turn the set screw at the base of the bathtub spout counterclockwise with a screwdriver and remove it.
Pull out on the bathtub spout to remove it.
Grasp the old nipple coming out of the wall, if it appears corroded, with a wrench and turn the nipple counterclockwise to remove it. Insert the new nipple and tighten it. If the nipple is too short to grasp with a wrench, insert an internal pipe wrench then turn it with the wrench.
Slide the new bathtub spout on. Tighten down the set screw with the screwdriver.
Turn on the water supply and test the unit.