There may not be a mess-free way to reheat spaghetti sauce, but you can try your best! Pop it in the microwave for 3-4 minutes and let it get nice and hot. Be sure to put a napkin over the top to help with any splatters. You can also pour the leftovers into a pot and heat it on the stove. Put it on medium heat and cook for 10 minutes, stirring frequently.