get the latest spamassassin deb:
dkpg -i the_spamassassin_file.deb
cd /etc/spamassassinif you log into the Plesk control panel now, SpamGuardian will still show that you are using the old version...
remove the old .cf files (I moved them to a nuke folder)
remove the Rules_du_jour files too
edit the local.cf file
check that everything is still working
fix any errors
Additional instructions can be found in various places.