Preheat oven to 425 degrees.
Make streusel topping by whisking together flour, sugar and cinnamon in a small bowl. Add melted butter and stir using a fork until topping becomes crumbly. Set aside.
Prepare blueberry muffin batter according to package directions with blueberry mix, vegetable oil, eggs and water.
Fill 12 prepared muffin cups two-thirds of the way full with batter. Top with streusel.
Bake 15-20 minutes.
Cool for 5 minutes before serving.