Store any leftover baked beans in an airtight container in the refrigerator. They should stay fresh for 3-4 days. To reheat, you can use the microwave or stovetop. Simply heat until they are warmed through, and you may want to add a little extra tomato sauce if they've thickened too much.
You can also store in the freezer for 3-6 months. When you're ready to use them, thaw the beans in the refrigerator overnight, and then reheat them on the stovetop or in the microwave.