Reheating means heating previously cooked food before serving it again. It is not a preservation method: leftovers must first be cooled and stored safely. Reheating is not a way to rescue food that has been stored unsafely.
For household leftovers, USDA recommends reheating to an internal temperature of 165°F (74°C), checked with a food thermometer. Stir or rotate food during microwave heating and check more than one spot to avoid cold areas.