How you grasp the retaining ring depends on which arm you are working on.

When you are working on the arm located near the on/off button, you must put your hand through the part of the headset that is normally towards your face to grasp the retaining ring. You then push the arm down in the direction shown in the second picture.

When working on the arm located near the USB port you must put your hand through the front of the headset to grasp the retaining ring. You then push the arm down as shown in the third picture.