updating question of spare hardware installed with opnsense

[skyeci2018]

Hi,

Been using opnsense for a little while and wondered if there is any way I can apply updates to my spare units which are for backup in case my main router goes down..

Is it possible somehow to apply updates as and when to these secondary box's across my main network without having to just re-install opnsense on these units from scratch to apply the latest updates?
appreciate this might not be possible but any ideas appreciated.

cheers