If you look at the front of the camera, stop down button is the square black button at the lower right of the lens.
The bad part of this situation is there is no access to the back of the lens from the back of the camera. The screws in the mounting flange are covered by the lens mount. So. Anything a technician can do, you can do.
I would try inserting a very thin screwdriver(1-2mm) or hobby knife blade between the edge of the tab(it's plastic) and the lens, and gently pry upwards while rotating the lens. The meter coupling ring on the body and may flex enough fo it to ride over the coupling on the lens.
If a repairman does that and breaks it, the lens is removed and the ring replaced. if you break it, the technician is one step closer to finishing the job.