You can reheat shells and cheeese in a preheated 300°F oven in a baking dish covered with aluminum foil. Place in oven for about 15 minutes. Check every 5 minutes after that. You might need to add an additional splash of cream, milk, or even water to loosen the consistency. You can also reheat in the microwave. The time will depend on how much you're reheating. Start at 1 minute and add 30-second intervals from there, stirring every minute or so.