As noted by Bon Appétit, the best way to reheat mac and cheese is the way you cooked it initially. If you made the oven-baked type of mac and cheese, then reheating it in the oven is the way to go.

You probably stored your mac and cheese leftovers in the baking dish they were made in, covered tightly with a layer of tinfoil. If so, all you need to do is preheat your oven to 350 degrees Fahrenheit, put the dish in, and reheat until hot and bubbly, about 20 minutes. Then, take off the foil and allow the dish to heat for another 10 minutes (via Insanely Good Recipes). Be careful not to overheat the mac and cheese, which will cause the proteins and fat in the cheese to separate, and which will also turn the pasta mushy.

If you made a stovetop version of mac and cheese, and you want to reheat it in the oven, it’s not ideal, but it will work. To do so, place some mac and cheese in a glass casserole dish, pour a bit of milk over the top, using one tablespoon of milk per cup of pasta, and gently stir it in. Cover tightly with foil and bake in a 350-degree oven until hot and bubbly, about 30 minutes.