broken window

I drive a 2002 325ci. The other day I was opening the rear side window when I heard a little pop and the button went limp. The button moves up and down, but doesnt do anything. How would I go about replacing the button ?

Hey all! Recentl;y I had to replace my rear window "raising" mechanism because the clips broke. Now I have a totally different problem with my front window. Basically it will go down at normal speed, but it will raise at a snails pace...sometimes it will stop half ways up, like it can't take...