Welcome to the site and the TM club.
Actually, under the cap is a bolt. You need a socket wrench. Once loose you can swing the arm.
Note, not sure where you are looking to move the arm or even which arm. But if you are looking to move the drivers side out and forward, you will be blocked by the front left windshield frame. Also, if you come out away from your coach to far, you run the risk of your mirror hitting a passing semi mirror. These are the reasons they are moved in closer to the coach.
Take care.