Lasagna is a bit time-consuming to make, but it reheats in a snap. Watch it closely so it doesn't dry out and add more liquid, if necessary. Let it rest for 5 minutes after you cook it to allow the cheese to set and the heat to evenly distribute throughout the pasta.
In the Microwave
Reheat small portions of lasagna in a microwave. Place the lasagna in a microwave-safe container and add a bit of marinara sauce or water to the container to prevent dryness. Cover it with a lid or a microwave-safe plate. Heat on high for one to two minutes, or until the lasagna is bubbly and a meat thermometer inserted in the middle registers 165 degrees Fahrenheit.
In the Oven
To reheat larger portions, preheat the oven to 425 F. Pour between 2 tablespoons and 1/4 cup water over the lasagna -- depending on its size -- and cover the lasagna with aluminum foil. Alternatively, add 1/4 cup marinara sauce to prevent dryness. Bake for 25 minutes, or until the lasagna is hot and bubbly. A meat thermometer inserted in the middle of the casserole should read 165 F.
Refrigerate leftover lasagna at 40 F within two hours of serving, and use the leftovers within three or four days. When foods are left at room temperature, harmful bacteria can grow. Discard the lasagna if it was left out.