FreeBSD has two branches that you will want to use: RELEASE and STABLE. The RELEASE builds are snapshots that are used for installation, such as on CD, USB or over the network. The STABLE branch is the latest updates by upstream applied since the RELEASE branch was created.
freebsd-update lets you upgrade between both minor (8.2 to 8.3) and major (8.2 to 9.0) versions, as desired.
Get the latest release of FreeBSD.
freebsd-update upgrade -r 9.0-RELEASE
If upgrading from 8.x, you may hit an error with the upgrade:
The update metadata is correctly signed, but failed an integrity check.
Cowardly refusing to proceed any further.
If this happens, update the script, and start again:
sed -i '' -e 's/=_/=%@_/' /usr/sbin/freebsd-update freebsd-update upgrade -r 9.0-RELEASE
This will run some scripts, ask some questions, and present some diffs. Continue as normal, and when finished, install the changes. Reboot, then continue with the install.
freebsd-update install reboot freebsd-update install
When upgrading between RELEASE versions, you will need to rebuild all of your progams installed through ports. Plan accordingly.