Before you cancel your contract, see if you can discuss the issues with your listing agent. Perhaps there is a way to "fix the problem" before starting your listing contract over with another agency.
If you still want to cancel your contract, ask your listing agent or the office broker of record to cancel your agreement. Keep in mind, unless you are unconditionally released, you may still be responsible for a sales commission to your listing broker. This is to avoid sellers gaining marketing exposure for their home, but then canceling the contract once a buyer is in the picture.
All the best!