Go to the junction box and unplug the light fixture from its socket. Attach a long piece of string onto it. Turn off the circuit breaker in case you have the wrong light.
Remove the old fixture from its sockets. In most cases, you will have to unscrew a small screw to remove it. If your light does not have a screw, it is probably one that snaps out of its socket. You will have to use your screw driver to pry open a tab.
Take the fixture to the surface. Keep pulling until the piece of string comes up out of the water. Untie it from the old fixture and tie it firmly to the new one.
Make sure that the new fixture is assembled correctly. Double check the position of the gasket and the lens, and see that it is tightly screwed together. Submerse the new fixture in water and watch for bubbles. If it bubbles, there is a leak. Take it out of the water, open it up, dry it off inside with a towel and carefully put it back together.
Put the light on the bottom of the pool near the socket. Go to the junction box and pull on the string until the cord follows the string through.
Screw the fixture into the socket. Go back to the junction box and plug in the fixture. Turn on the circuit breaker and the light switch to make sure the light comes on.