Store in an airtight container for 3-5 days in the fridge.
Freeze for up to 3 months. I personally flash freeze all my baked goods.
To reheat, place in a skillet with a splash of liquid (e.g. water, broth, sauce, etc.). Cover and heat over medium-low heat until warm, 2-3 minutes or so. This is to ensure meatballs don't get dried out.