It happens every year: you lug out the box of Christmas decorations, you pull out the strings of lights and then plug them in. Suddenly you find yourself with four or five bulbs that are dimmer than the rest of the lights on the string. This can be somewhat annoying---no matter the level of your Christmas cheer---if you feel that these dim bulbs detract from how nice your Christmas tree and your home look during the holidays. Believe it or not, the cure is relatively simple and the best solution is to replace the bulbs.
Take a piece of black electrical tape and mark the base of each of the lights that are dim.
Carefully remove the dim bulbs from your string of Christmas lights.
Make sure that you have replacement bulbs that are the same brand of Christmas lights as the string you are working on, and that your replacement bulb is actually a replacement bulb and not a blinker bulb.
Carefully center the wire on the bottom of the new bulb.
Line up the base of your new bulb with the slot inside the base of the socket, and gently push the bulb into the socket.
Plug in this strand of lights so you can be sure that they are bright enough. If they are, you've solved your problem. If they are still dim, it may be a short in the wiring in your strand of lights, and you may want to consider replacing the whole strand.
Remove the electrical tape you used to mark the sockets that had dim bulbs once you know the replacement bulbs are working properly.
Once you've fitted the new bulb in the socket, make sure it is snug. You don't want it to be too loose or too tight.
Buy an extra set of Christmas tree lights when you buy new lights just to have the right brand of replacement bulbs.
Don't throw away the bulbs you're replacing, because you may need them to help you get the right bulbs if you don't have replacements. Not all Christmas tree lights are created equally, so no matter how much two bulbs may look alike, they may not be alike or compatible at all.