Place leftover smashed burgers in an airtight container in the refrigerator for 3-4 days. If you want to freeze the burgers, wrap them tightly in plastic wrap, then place them in a freezer-safe container or a zip-top bag and freeze for 2-3 months. To reheat, microwave them or warm them up on a hot grill or in a frying pan for a few minutes until heated through.