Is it full HA (management HA and firewall HA on two boxes total), a VSX cluster, or a normal HA cluster?
For full HA, I would take a migrate_export (to restore to your current version if needed) and a 'migrate server' for the upgrade. I would then wipe the box, do a clean installation, and import the 'migrate server' file. This is pretty complicated. Management upgrades go wrong much more often than firewall upgrades do.
For VSX, I would use 'vsx_util upgrade' on the management, reinstall one member, use 'vsx_util reconfigure' on the management to reprovision it (same process you would use to replace a failed member), then repeat on the other member(s).
For a normal HA cluster, I would right-click the cluster in SmartConsole and pick Actions > Version Upgrade. Pick the version, hit "Install", and let it cook for about an hour. Out pops an upgraded cluster, though one without a jumbo. You can use Actions > Install Hotfix/Jumbo to handle that. This method is super simple, and there's no opportunity to forget a step and cause an outage.