A guy renting my downstairs has been late twice with the rent, and has broke the lease agreement more than 5 different times. He owes me money and refuses to pay. He just gave me 11 days notice. He still owes me money for August utilities. Am I allowed to change the locks?

No! Changing the locks is an illegal eviction.

You can give him a 3, 5, 7 or 10 day written notice to pay the rent or vacate the premises. The time depends on your state law. If he hasn't moved out or paid in full by the end of that time then you can file a lawsuit for eviction.