Start by Preheating oven to 325 degrees. If your oven is to how, it will make your meat tough and chewy.
Place brisket on a baking sheet while the oven is preheating and let it cool to room temperature before placing in the oven. You want the brisket to heat evenly, so you don't want to put cold brisket in the oven.
If you have saved any of your juices from cooking the brisket, pour it on the brisket before placing in the oven. If you did not save any, you can use beef broth.
Cover brisket with double foil and make sure it is sealed on the pan.
Place brisket in the oven.
For a whole brisket it will take about an hour to reheat. If it is sliced, it will reheat in about 20 minutes.