To replace a fuse in your Christmas lights, you'll need to locate the fuse box or the specific area where the fuses are housed. Once you've found the fuse box, you'll need to identify which fuse corresponds to the light strand that needs replacing. Typically, there will be a diagram or label indicating which fuse controls which light strand. Once you've identified the correct fuse, use a pair of pliers or a fuse extractor to carefully remove it. Then, take the new fuse and insert it into the socket, making sure it's properly aligned and seated. Test the lights to ensure they're working properly.