To reheat a burger in an air fryer, preheat to 350°F (175°C), then cook the patties for 3-4 minutes per side, flipping halfway. For a cheeseburger, add cheese in the last minute of cooking. Let the patties rest for a few minutes before serving on a fresh bun with your favorite toppings. This method ensures a deliciously crispy and juicy burger every time.