Summarize this article:

In case you've decided to make changes to your lights after you've put up your tree, you should remove the lights for safety and ease of access. Anytime you need to replace a bulb in a strand of lights, you should always find a replacement bulb with the same voltage as the original. Look under the tag on the strand of lights to find voltage information. Some light strands blink because they have special bulbs that twinkle. If that's the case with your lights, you can get rid of the twinkle feature by switching out the bulbs. Oftentimes, the bulbs that blink on these strands will be marked with red or silver tips. However, if they aren't marked, plug in the strand, let it warm up, and use a small piece of masking tape to mark each blinking bulb. First, unplug the lights. Then, for each blinking bulb, hold the bulb by the base (where the light attaches to the strand) and pull out the bulb and the bottom casing. Remove the bulbs from the casing. If you have trouble removing the bulb and casing, use a screwdriver to pry the light out. For every blinking bulb that you removed, you want to replace it with a non-blinking bulb. Insert the new bulb into the case, making sure the two wires on the end go through the holes in the bottom of the case.  Fold the wires back so they rest against the side of the casing. When you have finished, reinsert the new bulb and casing back into the base. When you have replaced all the blinking bulbs and returned them all to their bases on the light strand, plug in the lights and test them. Give them a few minutes to warm up.

Summary:
Remove the lights from the tree. Find replacement bulbs. Locate the blinking bulbs. Remove the bulbs. Replace the bulbs. Test the lights.