Upgrade in FreeBSD world means having a host on version 12.0 and doing similar procedure like in FreeBSD update, but ending up with a major or minor version number incresed, not patch version number.

TL;DR

env PAGER=/bin/cat freebsd-update upgrade -r 12.1-RELEASE
freebsd-version -ku
bectl create 12.0.11
freebsd-update install
reboot
freebsd-update install
reboot
pkg upgrade
freebsd-update install
reboot

freebsd-update utility will tell you if there is anything to be fetched. If there isn’t, just ignore the rest of the commands.

First, using freebsd-version and bectl you create a boot environment for the current version of FreeBSD. Then, first install will update only kernel. As FreeBSD kernels are backward compatible, your system can boot with newer kernel then the rest of the operating system. Second install will take care of FreeBSD base. If everything is OK, after another reboot, you should upgrade packages for the new OS version and run finall install which will take care of known package problems. The last reboot is there to ensure everything is working OK, as you might have some kernel modules, like drm-kmod, which are changed during the upgrade.