Turn off power to the transformer from the main circuit or unplug the transformer from the GFCI outlet.
Reach into the pool and remove the screw in the fixture's top edge using a Phillips head screwdriver. Place a flat-head screwdriver between the fixture's bottom edge and the pool's side. Wiggle the screwdriver to release the tab and free the fixture. Lift the fixture above water level.
Locate the nut that holds the clamp onto the fixture's top edge. Loosen the nut with a 3/8-inch socket wrench and remove the clamp. Slide the front cover off the fixture.
Insert a flat-head screwdriver between the rubber gasket and the fixture's lip to break the seal that holds the gasket in place. Lift the gasket off the fixture with your fingers. Remove the lens from the fixture. Reach inside the fixture and unscrew the light. Wear gloves when removing broken lights.
Dry inside the fixture with a paper towel. Insert the new light and twist it in place. Hold the lens in one hand and a new gasket in the other. Carefully work the gasket up over the lens edge.
Place the lens over the fixture and put the front cover over the lens. Position the cover with the screw hole on the top. When done correctly, the light cover's half circles point down toward the pool's bottom.
Wrap the clamp around the fixture's top edge. Tighten the clamp in place with a 3/8-inch nut and socket wrench. Tighten the clamp as much as possible.
Insert the fixture back into its niche, or hole, in the pool's wall. Make sure the tab on the bottom edge locks into the fixture's bottom edge. Insert the screw through the hole in the fixture's top edge and tighten it to hold the fixture in place. Restore power to the lights.