If you want to serve your Eggnog warm and have already stored it in the fridge, you can reheat it. The best way to reheat it is to pour the Eggnog back into a saucepan and then heat the mixture while occasionally stirring on low heat until the desired temperature is reached. If you are short on time, you can reheat the Eggnog in the microwave. Heat in 30-second intervals until the desired temperature is reached.