Cover leftover Peach Dump Cake tightly or transfer it to an airtight container and refrigerate for up to 4 days. Reheat individual servings in the microwave for 20 to 30 seconds or warm larger portions in a 325°F (165°C) oven for about 15 minutes until heated through. Freeze cooled portions in freezer-safe containers for up to 3 months. Thaw overnight in the refrigerator before reheating. To prepare ahead, assemble the peach filling in the baking dish up to 24 hours in advance. Add the dry cake mix and melted butter just before baking for the best topping texture.