It is actually really simple to update the installation to 1.4.5. This is how I did it. I'm not a sys admin. I just play one on forums, so YMMV.
Backup /etc/squirrelmail/config.php and /etc/squirrelmail/config_local.php - I usually make a directory called "old" and stuff 'em in there.
Go to where squirrelmail is: cd /usr/share
Backup original directory: mv squirrelmail squirrelmail.old
I link a directory for installs because I like directories that show me the version:
tar -zxf squirrelmail-1.4.5.tar.gz
ln -s squirrelmail-1.4.5 squirrelmail
I also like the idea of having all the config files in /etc - much easier to back up that way.
mv config.php /etc/squirrelmail/
mv config_local.php /etc/squirrelmail/
There is a "." at the end of the ln statements, so don't omit!!
ln -s /etc/squirrelmail/config.php .
ln -s /etc/squirrelmail/config_local.php .
Run conf.pl and set it up like you would normally. The only real gotchas are to remember to set up the IMAP server to be courier (option D to set) and to note the locations of the following directories(option 4 to set):
Sure it's a bit more work than an rpm file, but it gets you updated!