The E400 and E500 series of APs support two software banks: a primary image that is in use and a backup image that can be used if the primary goes bad for some reason.
When a firmware upgrade is carried out, the backup image is the one that is upgraded, while the primary image remains untouched, providing another level of reliability to these access points. When the AP is rebooted after the firmware upgrade, the new image is made the primary, and the previous primary is marked as a backup. Whenever the AP powers up it will check to see if it made it all teh way to runtime software. If it fails, the AP will reboot and try again, upto 8 times at which point it will flip partitions and activate the backup image.
This feature is especially useful when there is a power loss during a firmware upgrade: without the 2nd firmware image that woud have resulted in a partially flashed image, and requiring the administrator to take down and replace the access point.
Starting from version 3.2 the version numbers of the two firmware images on an AP can be seen in the AP GUI under 'Monitor->System':
One a new firmware release has been validated for a while in your network, it is recommended that an upgrade is done again, just to bring the backup firmwrae up to the same version as what you are using in production.
Sometimes for troubleshooting purposes there may be a need to force the activation of the backup image manually. This can be done using the 'service boot' CLI command: