Heat the oven to 350°F. The oven should be fully hot before baking.
Pour the peaches and all their syrup into the baking dish. Spread them into an even layer so the bottom is fully covered.
Sprinkle the dry cake mix evenly over the peaches. The peaches should be hidden under a dry, flat layer.
Cut the butter into slices and place them across the top. Try to cover as much of the dry mix as you can.
If using cinnamon, lightly sprinkle it over the top. You should still see the cake mix underneath.
Place the dish in the oven and bake until the top looks golden and the edges are bubbling, about 45 minutes.
Remove from the oven and let it rest for about 10 minutes. The filling will thicken as it sits.
Scoop into bowls or plates while warm.