You can reheat lasagna in an oven, convection oven, or the microwave. Ovens tend to take a lot longer than heating the pasta in a microwave, but there is less chance of ending up with an overcooked or dry dish. A microwave is best when you are reheating small portions and are short on time. In any case, you will need an oven-safe or microwave-safe dish to put the lasagna in, plus plastic wrap or foil to cover the dish, depending on which reheating method you have chosen. While there are ways to reheat a lasagna to its former glory, there is nothing unsafe about eating slightly overcooked or dry pasta if you accidentally overdo it.
To reheat a dish of lasagna in a regular oven, preheat it to 350°F (about 175°C). Use a fork to evenly poke holes all the way through the lasagna noodles. Carefully fill these holes with a total of a couple tablespoons of milk. If you or someone in your family cannot consume dairy products, water can be used instead. Cover the dish with foil, tightly sealing the edges, and then bake the lasagna for 20 minutes or until it is warmed all the way through.
Reheating lasagna in the microwave can be more complicated than reheating other types of food. Pasta tends to dry out when heated this way, so if you have leftover sauce, spread it across the surface to combat dryness. If possible, do not reheat it in the square dishes it is usually originally prepared in, because the lasagna can warm unevenly, favoring the corners. Instead, reheat it one slice at a time on a circular plate or other shallow dish. To successfully reheat lasagna in a microwave and avoid overcooking, microwave it for only 1 minute and follow that with 10 to 20 second intervals, checking how thoroughly warmed it is each time.
To use in a convection oven, follow the same steps used to heat it in a regular oven. Make sure to place the pasta in a shallow pan so that it can reheat all the way through. Be careful when opening the convection oven door, because there sometimes can be a burst of heat. In addition, do not leave the oven alone during the 20 or so minutes the lasagna is cooking. If you use foil to cover the lasagna when reheating it, make sure not to let the foil lay on the food itself, because the cheese will begin to stick to it once it starts to heat up.