As your not changing platforms or anything kicking off the upgrade itself is simple.
Read the release notes:
files.zimbra.com/website/docs/archives/4.5/Zimbra%20OS%20Release%20Notes%204-5-11.pdf Take a backup (snapshot/LVM/scp/rsync/etc). Tons of methods, a few are outlined here:
Backup and Restore Articles - Zimbra :: Wiki & there's more if you search the forums - but here's a simple one:
root
rsync -avHK /opt/zimbra/ /nas/backups/zimbra
su - zimbra
zmcontrol stop
exit
ps aux | grep zimbra
kill -9 loggerPID (it often keeps running on 4.5.x)
rsync -avHK --delete /opt/zimbra/ /nas/zmail/backups/zimbra
This second rsync with zimbra stopped covers any changes & the --delete parameter removes any files at the destination (/nas) that aren't at the source anymore (/opt/zimbra)
And if you wanted to take that one further:
tar -zcvf /nas/zmail/20081106.tar.gz /nas/zmail/backups/zimbra/
Then grab the 4.5.11 binary for your platform from
/community/downloads_previous.html
tar -xvf zcs-4.5.11_GA_1751.Platform.tgz
cd zcs
./install.sh
Follow the prompts
(If you're a single serer you probably
don't want to enable proxy; I forget what version added a 'are you sure?' prompt for turning it on.)