To store leftovers of a tater tot casserole, allow it to cool completely and then transfer it to an airtight container. You can store the tater tot casserole in the refrigerator for 3-4 days or in the freezer for up to 3 months. To freeze the casserole, wrap it tightly in plastic wrap and aluminum foil or place it in a resealable freezer bag. Label and date the container, and then put it in the freezer. When you are ready to eat the leftovers, thaw the casserole in the refrigerator and then reheat it in the oven or microwave until it is hot and bubbly.