How To: Removing sliding glass windows in a Classic Mini Cooper
Dig along the the bottom of the sliding window channel and you will find two or three phillips head screws. Get them out however you can - and this is sometimes difficult because the original screws are steel and they rust. You may have to drill the heads off. I use stainless ones at reassembly time.
Once the screws are out, slide a flat putty knife under the chrome channel inside the door and move it along from front to back. When it is free, the two windows, the chrome rail and the track will all come out of the door as one assembly.
Make sure you have a a good hold of the glass as the whole thing will eventually come out in a rush, and a glass can easily drop.