Note: this posting is more or less obsolete with regard to the current 5.x Mail::Toaster.
Here's how to upgrade ClamAV from ports:
1. Update your ports tree.
# cvsup -g /etc/ports-supfile
2. Check what version of clamav you are about to compile (just in case)
# cat /usr/ports/security/clamav/distinfo
3. Compile and install ClamAV
# cd /usr/ports/security/clamav
# make install clean
4. Restart the clamd daemon
# /usr/local/etc/rc.d/clamav-clamd.sh restart
# /usr/local/etc/rc.d/clamav-freshclam.sh restart
That's it.
If you want to install a version of ClamAV that isn't in the FreeBSD ports collection yet, fetch the tarball you want from http://www.clamav.net and follow the instructions inside the INSTALL file. I.e. ./config, make, make install, make clean. Then follow point 3 and 4 above.