Follow the upgrade instructions in the release notes.
Below is an overview, but still read the upgrade instructions in the release notes.
1. Backup the flash and copy it off of each controller
2. Upload the new version of software to the opposite partition that the controller is running on, but do not reboot any of the controllers. If you need to downgrade later you would just switch the partition.
3. Save the logs with tech support off the master controller, so you would know how many access points were active.
4. Reboot all your controllers at the same time.