When it comes time to upgrade a redundant system, it is easiest to upgrade the backup system first and then upgrade the master. Clients will continue running uninterrupted until the master is taken down for the upgrade.
WARNING: Be sure to create all necessary backups before performing any upgrade! Also be prepared for manually restart any clients after the upgrade, even if clients should be able to reconnect automatically after the upgrade (see below).
If you are upgrading from a pre-7.7 system to 7.7 or later, any running clients will need to be restarted manually. If you are performing a minor upgrade on 7.7 and later systems, the clients will be able to automatically switch between master and backup when the active node goes down for the upgrade. Major upgrades may still require manual client restarts.