To fix LED Christmas lights, start by checking the power supply. Make sure the lights are plugged into a working outlet and check the fuses or breakers. If everything looks good, try replacing the bulb or string of lights with a new one. If the problem persists, it may be an issue with the circuit board or wiring. It's important to follow the manufacturer's instructions when replacing any parts. You can also try to diagnose the problem by testing the lights one by one to determine which one is causing the issue.