Changing a fuse in Christmas lights is a simple process that can be done by anyone with a little bit of know-how. First, you need to locate the fuse in the light string. This can usually be done by checking the end of the string, where the cord meets the plug. There should be a small panel that you can open to reveal the fuse. Once you have located the fuse, simply remove it from its holder and replace it with a new one. Make sure to match the size and amperage of the new fuse to the old one, otherwise the lights may not work properly. After the new fuse is in place,