To replace a bulb in Christmas lights, use a replacement bulb from a similar string of lights, or purchase a pack of individual bulbs. Lightly pull the defective bulb from its plastic socket, and gently push the new bulb into place. Some lights require a slight turning motion.
When a string of lights does not glow, the first step is to determine which bulb is causing the outage. Find the defective bulb by removing any one of the dark bulbs and examining its leads. There should be two copper leads inside the bulb. Make sure these leads are in their proper positions by comparing them to those inside a functional bulb. Any bulb with missing or bent leads needs replacement. Discard the bulb, and replace it with an identical bulb.