This is from wikihow,
Use a pan on your stovetop. First, pour some cream. Bring it to a simmer. Stir your mashed potatoes into the cream evenly. Continue to stir until the potatoes are heated through. If needed, add more cream and wait for it to come to a simmer, then stir it into the potatoes.[1]
Depending on the amount of potatoes you’re reheating and the size of the pan, it’s best to start off with too little cream than too much. At the very least, use enough to coat the bottom of the pan.
Use a food thermometer to check the potatoes’ internal temperature; for health reasons, the potatoes should reach at least 165º before they’re safe to eat.[2]
I've reheated mash in a frying pan, with a little butter in the pan add the potato and keep moving it around until it's hot. You don't want to actually fry it.