Source: https://www.thespruceeats.com/top-ways-to-reheat-leftover-pizza-4173400

  1. Preheat the oven to 350 F.
  2. Place the pizza on a piece of foil and put it directly on the rack for even heating on the top and bottom. Alternatively, preheat a sheet pan as the oven heats up for a crisp crust. Add the pizza to the hot pan.
  3. Bake for about 10 minutes or until warmed through and the cheese is melted. If you'd like your pizza crispier, bake for longer. Note that if you’re using a toaster oven, you’ll only need to heat the slice for 3 to 4 minutes.