Combine the cake mix with the melted butter in a medium bowl until you have a crumbly mixture with some dry spots.
You can also sprinkle the cake mix on top of the peaches, even it out carefully, and then pour butter on top, covering as much as possible. I don't do it this way because sometimes the topping is baked with large floury patches that never quite mix with the butter. Alternatively, place thin slices of cold butter over the dry mix (see the 3-ingredient dump cake for reference).