If you did regular cluster upgrades, this is not that much different. So, you would follow this logic...ALWAYS upgrade whichever standalone fw portion is backup (mgmt part is not that important, as it does not process the traffic, so even if that one is active, it will become standby mgmt upon reboot), uncheck "do not install..." option when backup fw is upgraded to new version, once rebooted, then follow same process for current active standalone fw part.
Hope that helps.
Remember this, as some people get confused about it...when it comes to mgmgt part, primary is ALWAYS primary and secondary is ALWAYS secondary, but either one can be active or standby, so secondary can be active or standby, thats fine.
Best,
Andy