You don't need to use Firepower Chassis Manager.That's only necessary if you want to provide an alternate boot image for the hardware manager to use when initializing the SSP from scratch.
You can just log in via ASDM and upgrade just as if it was a traditional ASA. (upload to both appliances, change boot image line, reload standby, make active and reload previously active unit.) It is a zero downtime change but we still recommend having an approved maintenance window to manage your stakeholders' expectations of an unlikely but possible service disruption.
