1. Preheat oven to 350°F/180°C.
2. Have ready an 8x11-inch ceramic or glass dish.
3. Dump the canned peaches with the syrup into the dish, spreading to cover the whole pan. 
4. Sprinkle with cinnamon. 

For the topping:

1. 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).
2. Top the peaches with this mixture, smoothing it slightly but leaving the edges uncovered. It will not cover them completely and that is fine. Don't stir or mix it with the peach filling, simply smooth it out a little. 
3. Bake for 40-45 minutes, until golden and dry and the filling bubbles around the edges, checking close to the end of the baking time by carefully lifting the topping to ensure it's fully baked inside. Bake it more if necessary.
4. Serve warm and plain, with whipped cream or a scoop of vanilla ice cream (the best way in my opinion). 
5. Store leftovers covered or in an airtight container in the refrigerator. Warm slightly before serving (in a medium-low oven).