You can just chuck the CD in and boot. It will upgrade itself with minimal involvement.
Given that you are using 5.5 that is pretty much it (earlier versions apparently need an intermediate upgrade first)
I have never backed up properly, and never had a problem, of course I would advise that you back up very thorougly first

No telling what state you could leave your system in if the upgrade should fail...
Why not get the new Contribs.org 6.0.1 release
http://no.longer.valid/news/article.php?storyid=16 it contains bug fixes over the older Mitel 6.0 version (and a pretty new logo

)