Preheat oven to 350 degrees F. Line a shallow roasting pan with foil, if desired. Spread popcorn in the shallow roasting pan. In a small bowl combine sugar and cinnamon. Lightly coat popcorn with cooking spray, tossing to coat evenly. Sprinkle with cinnamon mixture, tossing again to coat evenly. Bake about 5 minutes or just until crisp.