This we be quick. We chefs are often asked how to reheat pizza. Obviously, pizza does not need to be reheated (and we certainly have a soft spot for this thermally-deprived dish), but should you choose to reheat it, here are two ways to go about it
The starch molecules that make up a large part of the breaded portion of the pizza will have started to retrograde—this is just a fancy way of saying that they start to form structurally-sound units that make the bread tougher (this is what causes bread to go stale). To get rid of that stale texture, you need to heat the starch enough to break-apart/melt those structures. This is why you can make stale bread enjoyable by toasting it: the toaster melts those tough parts of the bread, making it soft again.
The Slow Way—Requires oven (ideally, though optionally, with pizza stone)
Preheat your oven (and stone) to its highest setting. Once preheated, place your cold pizza in the oven and remove it once the cheese begins to look all good ’n’ melty (should only take a few minutes). This is the slow way because you have to let the oven preheat.
The Fast Way (if you are only doing a couple of slices)—Requires a frying pan and oven
This is one of our favorite methods because it softens the inside of the crust (in baking terminology, this is called the crumb), and simultaneously gives the bottom a satisfying, wafer-thin crunch. Turn your oven on and set it to broil. Get a large, stainless-steel pan hot (a splash of water should bead up and skitter across the surface), and put your slice in the pan. After a few minutes, remove the slice and place in the oven (middle or top rack) and broil until the cheese begins to bubble.