Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch baking pan.
Mix cake mix, water, eggs, graham cracker crumbs, butter, and cinnamon in a large bowl; beat with an electric mixer on medium speed until batter is smooth, about 2 minutes. Fold apples into batter; pour into the baking pan. Sprinkle brown sugar over batter.
Bake in the preheated oven until cake pulls away from sides of the pan, 40 to 45 minutes.