You have to take the whole thing out with mechanism still attached , it's a pig as the glass part of window runs on a metal slotted runner with 2 small wheels running in it, one each end of a scissor arm.
Once you undo about seven or eight 10mm (spanner/socket - 6mm thread) bolts (one is under a small round polythene plastic cover thats stuck on,its the rear most one on each side .
Once you undo the whole lot you can use the motor switch to move the arms up and down . Once you have one off it's just a matter of sliding it off the other .
Dont ask about refitting..........................