Hi Alexy
You can start the upgrade by logging into either/any node's GUI, it does not matter if it is the follower or leader.
The system will first update the data and then begin upgrading the firmware on each nodes. The system should continue controlling the AP's and providing service until all nodes are upgraded and the cluster is restarted. AP service to clients using PSK or open authentication should not have service interrupted but services provided through the SZ like captive portal and proxied authentication will be interrupted when the cluster reboots.
AP code does not get upgraded until you select to do so per AP zone. The exception to this is SZ-100 and vSZ-E prior to 3.5 where there was only a single zone architecture.
The upgrade procedure was enhanced in version 3.5 and above so that node firmware is actually upgraded at the same time decreasing the upgrade time and providing a more comprehensive upgrade progress in the GUI
I hope this answers your questions
Albert
