Preheat the oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large bowl, mix together flour, sugar, baking powder, baking soda, salt, and cinnamon.
In another bowl, whisk together melted butter, eggs, and milk until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in the diced apples until evenly distributed.
Fill each muffin cup about two-thirds full with the batter.
Bake in the preheated oven for 20-25 minutes or until a toothpick comes out clean.
Remove from the oven and let cool in the tin for 5 minutes.
Transfer muffins to a wire rack to cool completely.
Enjoy warm or at room temperature!