Jaids,
The retaining ring isn't your problem.
Grab the switch cover between your finger & thumb, pluck it out of it's retaining grove.
In the center of the push button is a hole leading to an allen screw, it's about a 2mm/ 5/64.
Loosen the screw, click the button to the down position, slide the whole assembly out the tail end.
The retaining ring comes out easily with a screw driver and patients.
Theepdinker