How do I upgrade spamassassin 3.0.6 to 3.1.8

[Van G]

I'm a newb with limited command line knowledge and a FC4 virtual server with Plesk 8.1.1

I need to take control of the spam and virus passing through our user accounts.

Could someone give me a push in the right direction and help me achieve the above?

Currently running 3.0.6fc4
The contents of my local.cf:
report_safe 0
rewrite_header subject *****SPAM*****
required_score 7