I think it's :
Tilt glass as far up as possible
Shift the little tab to the 'other' side (so if it's at the inside, you slide it outside - or vice versa). Use a thin screwdriver to get at it.
Work the mirror glass off its mountings (ideally pull off squarely). It will have heater wires (black iirc) and the photochromatic wires too, which have an 'awkward to reach' connector towards the outside front of the casing.
Here's a picture of the glass carrier back:
Note that the lockring works on four tabs on the tilting mechanism in the mirror housing. These locate in the four apertures shown by the four small red arrows. When open, the tabs can come out.
Lock ring in locked position
Lock ring in open position:
Please note that the plastic is broken at the bottom of the carrier, near where the locking tab is located.