hope this helps someone , on the first one I ended up breaking a trim ring , before I finally got it .
so first I used some penetrating fluid on the back side of the trim ring , this will at least "loosen" some of the road grime usually backed in the gap . washing it with a hose might work as well
you can see the mirror glass all scrached up
the OE low mount mirror glass is held in with 2 screws and a trim ring
phillips screwdriver will remove the 2 screws
now , with the screws removed , you need to push up first on the top to release the "tangs" holding it in place
then you can do the same for the sides .. TANGS = 2 top , and 2 on each side and then the trim ring will come off
you can see the "tangs" in these pics
I just went to a glass shop and had them cut me a new mirror for 5 bucks .. on the original mirror , you can see it has some "tape" on the back side .. This is to stop it from rattling , so on the new mirror glass , I just used some double sidded tape of the same thickness
set the glass in place
and "snapped" the ring back in place , and installed the 2 screws ..