DETERMINE IF LIGHT FIXTURE IS BAD - If your light continually trips the GFI circuit, there's a good chance that it has developed a short in the light fixture or wiring. At this point you will have to replace the whole light fixture and wiring. If you have two lights tied together than you will have to wire each light independently to test which light is tripping the GFI.
DISCONNECT LIGHT WIRES - Locate the power junction box for your lights. It will normally be next to the house close to the pool control box. Open up the junction box and disconnect the wires to your light You may have several light cables coming into this box. Generally they are labeled on the PVC pipes. If not, you may have to disconnect each independently and turn the power back on and off to see which cable powers each light. Mark the pipes for the future.
ATTACH CORD TO POWER CABLE - Once you have determine which cable you want to pull out, attach a cord or special fish tape to the wire. One way to do this is to bend back the white and black wires to form a loop. Leave the green ground wire out straight. Then attach the cord though the loop. The green ground wire will parallel the cord as shown.
PULL LIGHT CABLE OUT OF WALL - Pull the light cord out of the pool wall. There is a conduit between the pool and the J-Box at the house. When you pull out the light cable at the pool, you will be pulling the cable out of the conduit and pulling through the cord that you attached to the other end