You can reheat a precooked, spiral ham by placing about ½ cup of water in the bottom of the pan and covering tightly in foil, and then heating for 10 minutes per pound at 350° F (180° C).
If reheating in the microwave, pour a little liquid in and loosely cover sliced ham with plastic wrap or a lid and heat on high for 1 minute, turn the dish, and heat in 1-minute intervals until reheated.