Stick it in the microwave
Place the mac and cheese into a bowl (microwave-safe, of course) and add 1tbsp of milk for every cup of mac and cheese. Cover the bowl with an inverted plate. Don’t forget to put on your oven mitts so you don’t end up burning yourself. Place the bowl inside the microwave and let it heat on low for a minute. Then stir, and keep heating it for 30-second intervals until it reaches the preferred temperature.
In the oven
The microwave is fast, and it's easy, but it cooks quite unevenly. There will likely be spots that are still cold and some that are hotter than the surface of the sun. An easy, reliable, though a bit slower, way is to reheat it in the oven.
Preheat your oven to 350 degrees and put your mac and cheese into an oven-safe vessel, such as a casserole dish. Butter the sides to prevent sticking and bonus flavor. You can even add a breadcrumb topping if you'd like: toast up breadcrumbs with salt, and herbs, mix with cheese, and sprinkle over the top if you'd like to go extra gourmet. If plain is fine, then just stick the macaroni into the oven and bake for around 15 minutes to reheat. Your mac and cheese will be as good as new and not separated and greasy like it would be if you had used the microwave.
In a pan
If you don't want to wait for your oven to preheat, you can reheat macaroni in cheese in a pan. Simply set your pan to medium-high and add a bit of oil or butter to grease. Then, add your macaroni and cheese. This is now a great time to add something from your fridge to spice it up: a bit of grated cheese, maybe some leftover meat, or some seasonings. If you want to add uncooked meat, cook it before you add your macaroni. Let sit for a minute or two in the pan, and stir to combine and ensure even cooking.
