Christmas lights are a real nuisance to repair. The hard part is finding the bulb that is burnt out and most people do this by trial and error. If you have a string of LED lights that are of the non-replaceable type you may think all is lost. But don't be too hasty. What if you could find the problem LED and then repair the string. Let me illuminate the solution.
You will need some tools that most people who tinker with electricity will have.
Step 1: WARNING
The following involves working with electricity. Electricity can kill you if you don't know what you are doing. So if you are not qualified to repair devices that require electricity then don't attempt the following repair. If you do follow these instructions then you take full responsibility for getting electrocuted - which as mentioned previously can kill you - or for any other risks associated with the activity outlined in this set of instructions.
For example, if you burn yourself with the soldering iron that is your fault. If you get lead poisoning from eating the lead solder this is you fault. If you burn down your house because you did not insulate your repair properly this is your fault. If your spouse whacks you with the rolling pin because you destroyed his/her Christmas lights guess what - your fault.