Stir sugar and cinnamon together in a small bowl; set aside for topping.
Warm apple pie filling in a skillet. Add flour and butter. Simmer 2 minutes to thicken, then cool.
Separate biscuits. Press each into a 5-inch round on a lightly floured surface.
Spoon a heaping tablespoon of filling onto half of each round. Brush edges with water.
Fold to close. Crimp with a fork to seal tightly. Cut a small vent on top.
Heat oil in a deep skillet to about 350°F over medium heat.
Fry pies in batches until deep golden, 2–3 minutes per side. Do not crowd the pan.
Drain on a rack. Sprinkle generously with cinnamon sugar while warm. Rest 5 minutes before serving.