How do you repair cracks and chips in the fiberglass of a hot tub?

If it is gelcoat finish just clean out the chip, get all the loose material out, get a tiny bit of gel coat, add a few drops of catalyst and fill it in. Tape over it if need to hold it in place, or get a heat gun and blow on it a bit til it starts to dry. Then sand with wet/dry paper to get it flat. Buff if needed.