Upgrade in CBSD means the same as in FreeBSD: increse only in patch version.

cbsd baseupdate
service cbsd restart

For upgrade you need to stop the jail, set it to new base and start the jail.

cbsd jstop nginx
cbsd jset jname=nginx ver=12.1
cbsd jstart nginx
cbsd etcupdate jname=cbsd mode=update from=12.0 to=12.1 mode=diff

CBSD will ask you how do you want new base files to be fetched, and default is to download them. Other options include compiling from code and using host system files as new base. If you upgrade more then one jail, the first jstart will create base jail for the rest to use.

Once you’ve upgraded all your jails, it’s time to cleanup

cbsd removebase ver=12.0