The no downtime upgrade can only be done on Active/Passive clusters because Active/Active clusters are both always recieveing traffic from their upstream load balancer.
If you have NSM you can do the upgrade from there. Othewise the procedure is outlined in the FAQ
How do I upgrade a cluster?
Answer: If you have central manager, enable all nodes and just upgrade one node of the cluster. After the node upgrades it detects the nodes that run the older version of the software. It instructs them to upgrade as well.
If you do not have central manager you should open multiple admin sessions, one for each node in the cluster. Then perform the upgrade by pressing the button almost simultaneously in all the windows. The nodes will upgrade, reboot and rejoin into one cluster. If you upgrade only one node, it will reboot, find the other nodes running an older version of the software, wait for about two minutes and eventually boot as a disabled node. Attempting to enable the node will silently fail with a log message in the events log. To continue, disable all the nodes and then enable the node you have upgraded. Then upgrade each node independently and enable it after upgrade.
Central manager license is strongly recommended in particular in clusters with more than two nodes.
Steve Puluka BSEET - IP Architect - DQE Communications Pittsburgh, PA (Metro-Ethernet & ISP) - http://puluka.com/home