Each Cisco WLC can boot off the primary, last-loaded OS image or boot off the backup, earlier-loaded OS image. In order to change a Cisco WLC boot option, issue the config boot command. By default, the primary image on the controller will be chosen as the active image.
Examples
> config boot primary
> config boot backup
When you upgrade the WLC with a new image, the WLC automatically writes the new image as the primary image and the previously existing primary image is written over the backup image.
Note: The previously existing backup image will be lost.
In order to see the active image that your controller is currently running, click on Monitor from the WLC GUI and look at the Software Version field under Controller Summary on the controller GUI. From the CLI, you can issue the show boot command to view the primary and backup image present on the WLC. Here is an example:
(Cisco Controller) >show boot
Primary Boot Image............................... 4.0.179.8
Backup Boot Image................................ 4.0.206.0 (active)
In order to remove or overwrite an image on the WLC, boot up the WLC with the image that you want to keep and perform an upgrade. This way, the new image replaces the primary image.
Source: https://supportforums.cisco.com/thread/2056217?tstart=0
