Hi
On my side, I would force the standby becoming Primary, then reboot the actual primary, when it's back up, force a switchover to give back primary role to your rebooted asa and you'll be able to reboot the secondary.
By doing a force failover you can ensure that the other one is taking the traffic compared to rebooting the primary and if something goes wrong, you'll have a downtime.
Thanks
Francesco
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