How to apply updates on OpenBSD, NetBSD, and FreeBSD?

OpenBSD is binary-centric. You can update the binaries (if any updates/changes are available) by executing pkg_add:

pkg_add -Uu

The OpenBSD team recommends using the packages over building from ports - The OpenBSD packages and ports system

FreeBSD can be updated via packages or ports.

In general, when using OpenBSD you only update your packages when you update your system. So, as a final step, after upgrading to the latest release, you should execute:

 # pkg_add -ui

Which will (u)pgrade your installed packages asking you any questions (i)nteractive when needed.

In general, packages for a given release are not updated until the next release (OpenBSD lacks the developer resources for providing updates to packages on versions other than 'current'). If you want to upgrade your packages more regularly, you either need to use ports, or upgrade to a new snapshot, and then run pkg_add -ui again.

For further reading, check: http://www.openbsd.org/faq/faq15.html

OpenBSD: M:Tier provides stable packages for the amd64 and i386 architectures. This allows you to keep your system up to date with the most recent security fixes from the -stable ports branch.

An article on OpenBSD Journal explains the details:

In practice, this means that as soon as a security fix/update is committed to the OPENBSD_5_3 tree a package will be built from the CVS tree. This package is then being tested and pushed to our fan-out server over at Stable.MTier.org, for everyone to use!