The best way to reheat already baked pies is to place them on a baking sheet and reheat them in the oven. This helps keep the crust nice and crisp, while evenly reheating the filling. The details will depend on your pie (type and size), but typically your oven should be set between 200°F and 350°F for warming. Of course, if you just want to quickly heat up a single slice of pie for yourself the day after Thanksgiving, 10-20 seconds in the microwave should do the trick!