FreeBSD does not include a port upgrading tool, but it does have some tools to make the upgrade process somewhat easier. You can also install additional tools to simplify port handling.
The pkg_version(1) command can generate a script that will update installed ports to the latest version in the ports tree.
# pkg_version -c > /tmp/myscript
The output script must be edited by hand before you use it. Current versions of pkg_version(1) force this by inserting an exit(1) at the beginning of the script.
You should save the output of the script, as it will note packages that depend on the one that has been updated. These may or may not need to be updated as well. The usual case where they need to be updated is that a shared library has changed version numbers, so the ports that used that library need to be rebuilt to use the new version.
If you have the disk space, you can use the portupgrade tool to automate all of this. portupgrade includes various tools to simplify package handling. It is available under sysutils/portupgrade. Since it is written in Ruby, portupgrade is an unlikely candidate for integration with the main FreeBSD tree. That should not stop anyone from using it, however.
If your system is up full time, the periodic(8) system can be used to generate a weekly list of ports that might need updating by setting weekly_status_pkg_enable="YES" in /etc/periodic.conf.