Yes, you can. First you'd need to download the 2.5 Recovery CD ISO images from:
http://www.cisco.com/cgi-bin/tablebuild.pl/wlan-sol-eng and then create the CD following the README file instructions. Once done, follow the section 'Upgrading from the Recovery CD' at"
http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/cwparent/cw_1105/wlse/2_5/inst_gd/ig_soft.htm#1104429
Note: This procedure will destroy all data and install a new image. You will need to replace the data by restoring a backup. For information on backups, see the online help or the User Guide for the WLSE 2.5 on CCO..