If you want to reheat frozen lasagna, you need to first thaw it. Let the lasagna thaw overnight in the fridge. You can reheat it using any of the methods mentioned above, although reheating lasagna in the oven will give the best results. If you have forgotten to thaw the lasagna overnight in the fridge, you can set it in the microwave and select the defrost option. You can also directly heat the frozen lasagna in the oven but know that it is going to take a longer time to heat up.