Couple things you should have set up first. This is how I do it, you may get different responses from different members.
1. Download the OS for your device. Latest Operating System Available For Each Blackberry Device
2. download desktop software. You can get this on the blackberry site.
3 install both on a separate computer. (not the BES)
into BES and choose to do a wipe device. (this will remove everything including OS)
5 unplug, wait 30 secs than pull battery and replace.
6 on the desktop with the software installed, goto c:\program files\common files\Research in Motion\Apploader\ Rename the Vendor.xml file to vendor.old.
7 start the desktop software
8 it will ask you if you would like to update/install.