Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking dish. Prepare the cake mix according to package directions, pour into the dish, and bake as instructed. Let it cool on a wire rack for 15 minutes.

While the cake cools, drain the peaches, reserving 1 cup of the heavy syrup. Chop the peaches into small pieces. In a small bowl, whisk together the reserved 1 cup of peach syrup and the sweetened condensed milk.

Using the handle of a wooden spoon, poke holes all over the top of the warm cake. Slowly pour the syrup mixture over the cake, ensuring it gets into the holes.

Scatter the chopped peaches over the cake. Gently spread the thawed whipped topping over the peaches.

In a separate bowl, combine the flour, brown sugar, and cinnamon. Cut in the cold butter until the mixture resembles coarse crumbs. Sprinkle the crumble evenly over the whipped topping.

Cover the cake and refrigerate for at least 4 hours, or preferably overnight, before slicing and serving.