Preparation: Begin by preheating your oven to 350°F (175°C). Prepare a 13×9-inch baking dish by greasing it and lightly dusting it with flour.
Mix the Batter: In a large mixing bowl, combine the cake mix, ground cinnamon, sour cream, eggs, and water. Beat with an electric mixer until the mixture is smooth and all ingredients are fully integrated. Transfer the batter to the prepared baking dish.
Add Pie Filling: Distribute the apple pie filling evenly over the batter by spooning it one spoonful at a time across the surface.
Bake the Cake: Place the dish in the oven and bake for about 35 minutes, or until a toothpick inserted into the center of the cake comes out clean. Once done, allow the cake to cool for about 10 minutes.
Prepare the Glaze: In a small bowl, whisk together the powdered sugar and milk until smooth. Drizzle this glaze over the cooled cake. Allow the glaze to set completely before serving.