pkg_add should suffice. You can also use
pkgin which is a much nicer package manager, if it's installed.
If you don't care about building from sources, you can skip
sysbuild and just do
sysupgrade auto, which will download and install pre-built binary sets - no compiling necessary.
EDIT: A word of warning. If you're upgrading within a major release (e.g. 9.1 -> 9.2) then
sysupgrade auto is fine. However if you're upgrading major releases (e.g. 8 -> 9) or CURRENT, then you have to first upgrade the kernel, reboot, and then extract the userland sets.