As tone said all you have to do to get the mirror off is turn the whole unit one way or the other and then it slides off. Moncha is right, in order to get the old mounting braket off, the windsheild must be heated to extreme temps and the guy I asked about it who has been doing it for years said that 4 out of 10 times you try to remove it by doing so it will ruin the entire windshield. So what I had them do (when replacing the old Vulture shattered windshield) was to take the old mounting unit off of the old rear view mirror and attach it higher almost to the very top of the windshield, while leaving the old mounting bracket in place. You would be surprized you can't even really see the old one below the new one even though there are two of them on there now.
AND YES IT IMPROVES REAR VISIBILTY A LOT!!!!! Oh my gosh I am so glad I had them do that, I can see cars behind me at red lights now in my rear view mirror!!!!! It is worth having two of the mounting brackets on there. And yes it is so high now it does kinda sometimes interfer with the sun visors, but you can easily move the rear view mirror down a little when you do need the visors (which I have never used because I am tall and they don't help that much anyways). I will try and post some pics of it soon so you can see what I am talking about.
In Christ,
Ryan