Mashed potatoes that have been refrigerated for about 5 days or less can be reheated on the stovetop or in the microwave. Reheating mashed potatoes on the stovetop: Place mashed potatoes in a saucepan over low heat. Stirring occasionally, and heat until the mashed potatoes are hot all the way through. Reheating mashed potatoes in the microwave: Place mashed potatoes in a microwave-safe dish and heat on