From Popular Science:
PopSci is based in New York City, so we tested each method with the thin-crust style of pizza the Big Apple is famous for. If you’re reheating Chicago-style deep-dish pizza or another variety, your results may vary.
The slices we used (both plain cheese and topped with pepperoni) spent 12 to 48 hours in the fridge. Reheating frozen pizza is a whole other ballgame, and we can’t speak to that here.
The official reheating method of the /r/pizza subreddit, this calls for placing your cold pizza on a non-stick pan and cooking it for two minutes over medium-low heat (or until the bottom of the slice is crispy). Then, pour two drops of water (less than a teaspoon) into the pan as far from the pizza as you can get. Cover the pan with a lid and turn the heat to low. Cook it for another minute.