Preheat oven to 300º. Coat a large baking pan and a large mixing bowl with cooking spray. Place popcorn in the bowl.
Combine syrup, butter, salt, and cinnamon over medium heat. Bring to a boil, stirring only until combined. Cook without stirring for 2 minutes. Pour syrup mixture over popcorn in a steady stream, stirring to coat. Spread popcorn mixture in an even layer in the pan. Bake for 15 minutes. Allow to cool completely in the pan.