If you need to replace a fuse in your Christmas lights, follow these steps: 1. Make sure the lights are unplugged from the power source. 2. Locate the fuse holder on the light strand and carefully remove the old fuse. 3. Insert the new fuse into the holder, making sure it is facing the correct way. 4. Plug the lights back in and check if they are working properly. If they still don't work, there may be a more serious issue with the lights that needs to be addressed.