Upgrading a system is just one step: pkg update.
As a prerequisite, make sure the system can access all required software packages. As a followup, you might in some cases need to do more than one pkg update.
Use the --be-name option to give the new BE a meaningful name.
Specify the result you want: Specify part of the version of the packages that you are updating.
$ pkg update --be-name 11.4.0 email@example.com
In some cases, image upgrade requires multiple steps because a subset of software must be updated before other software can be updated. If you are not able to upgrade to your desired end state in one step, update as far as you can, reboot to the new BE, and update again. Release notes provide information about whether a multi-step upgrade is required.