If it is a whole butt I'd remove it from the bag and cut it into smaller pieces so as to make reheating more even. Put the pieces in a pan, drizzle with a little juice, stock, vinegar or a combo, cover the pan with foil, then reheat in a 300 oven till hot, stirring once after 20 min and again 15 min later. (If it is a partial--just a piece--you can do the boiling water thing. Bring the water to a boil, add the bag, cover the pot, let it go for a several minutes, then shut the heat off and let it sit 20 min or so.)