Pour both cans of peaches (with juice) in a 9×13 glass baking dish. You can cut up the peaches into smaller pieces to make them easier to spread over the whole dish.
Sprinkle cake mix over peaches until all are well covered and you've used the entire box of cake mix.
Cut butter into 16-18 slices and lay on top of cake mix in rows.
Sprinkle brown sugar over the top, covering the entire cake evenly.