Sticky: HOWTO: Install or reinstall BlackBerry OS
http://www.blackberryforums.com/viewtopic.php?t=2412
Also, please note the exclusions for the OS4.0.
AT&T
4.0 IS NOW AVAILABLE! - Download Here (Apps 4.0.0.171)
(Except 7290)
Cingular
4.0 IS NOW AVAILABLE! - Download Here (Apps 4.0.0.171)
(Except 7290)