When reheating refrigerated or frozen leftovers in the oven, reheat them at the original cooking temperature; using a higher temperature could result in dry, overcooked foods.
Always use a preheated oven for reheating. Transferring frozen food in a ceramic or glass baking dish directly from the freezer to an unheated oven could cause the dish to break.
If reheating a dish in a microwave, use medium power. If reheated on high power, the edges of the food could overcook before the center is heated.