Download the image from support.arubanetworks.com
Then you have two options:
1) Upload through the web GUI
Go to Maintenance > Image Management and look to see which parition is "default boot".
Then upload the new image to the other partition using the "local file" option.
2) Via SCP or TFTP server
- Copy the image to the server
- From the CLI issue, show image version. This will tell you which image is active (default boot).
- For SCP: copy scp: <server ip> <username> <filename> system: partition <opposite partition number>
- For TFTP: copy tftp: <server ip> <filename> system: partition <opposite partition number>
- write mem
- reload