ArubaOS 6.4.3.7 is the latest GA code, and that is what you should use.
You would typically
- Identify any current issues with the customer's network. If they are bad issues, they should be troubleshot until resolved, before your upgrade.
- Upgrade the master and backup master
- Upgrade all of the other controllers at the same time, if possible.
If you want to upgrade them all at the same time, it is important that you do not have any access points rebooting, because having the master and any of the locals on a different version of code could cause a bootloop, where rebooting access points will not recover until all controllers are on the same version of code.