Spring pressure holds it on.  Pry up on the opposite side of the arm.

Pry off the plastic cap. Pull the arm up slightly until the holes are
aligned. Place a small nail through the holes. Loosen the 10 mm nut until it
is slightly above flush to the stud. Place fingers around the stud and pull
hard. Tap on the nut with a small hammer while pulling if necessary .

When replacing the arm, be sure to hold the arm up with a nail in the holes.
Use blue loctite on the threads.