To reheat the entire quiche, place the dish in a cold oven (not preheated yet). Then set the oven to warm up to 325ºF and set a timer for 30 minutes. By the time the timer goes off, the center should be heated through again. To reheat an individual slice, you can place a slice in your air fryer for 5 minutes while it preheats to 350ºF, or you can microwave for 45 seconds on high.