Reheating a precooked ham should be done at a moderately low temperature for between 15 and 30 minutes per pound, depending on the style of ham and whether it is boneless or bone-in. It is imperative to monitor a precooked ham closely so as not to dry it out by overcooking it.
An example would be a traditional boneless ham. This ham heats at 325 degrees Fahrenheit in a baking dish with water for about 20 to 30 minutes per pound. The United States Department of Agriculture recommends that all hams be reheated at no lower that 325 degrees. All hams should be heated until an internal temperature of 140 degrees is achieved. Another type of ham, such as a spiral-sliced boneless ham that weighs 9 pounds, should heat at 325 degrees for 2 hours. When removing any ham, tent it with aluminum foil and let it rest for about 10 to 15 minutes before serving.