On the Stovetop - Use a small saute pan, over medium heat, and add a little milk to make it creamy again. This is my favorite way to reheat leftover mac and cheese. In the Microwave - Add a little milk as above, and reheat for between one and two minutes. In the Oven - Place in an oven-safe dish, stir in a little milk and cover with foil. Reheat for 15 minutes at 300° F.