Drain the spa. Open the drain valve on the spa and allow the water to drain completely.
Wipe off the light cover and the area around the cover with the towel.
Twist the light cover from the spa. It will be either plastic or glass and should unscrew so you can access the light bulb inside of it.
Unscrew the light bulb from the housing inside the spa's light fixture. Screw a new light bulb in the light socket. Make sure the new bulb is the same make as the original bulb.
Screw the light cover back on the light and make sure it is tight so no water can seep inside of it. Turn on the light to test it. If it works, fill the spa with water.