Preheat oven to 350°F (175°C). Spray a 9×13-inch baking pan with baking spray.
In a large bowl, combine cake mix, oil, water, and eggs. Beat with an electric mixer until smooth.
Fold in the chopped apples until evenly distributed.
Pour the batter into the prepared baking pan and spread evenly.
In a small bowl, mix together sugar, cinnamon, and melted butter until combined.
Sprinkle or gently spread the cinnamon topping evenly over the batter.
Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean.
Let the cake cool slightly before slicing and serving.
Enjoy!