I agree with Ralph,
I would not want to do an upgrade on a business critical system unless i had support on standby, 9/10 times you don't get any problems but is it worth the risk?
The upgrade process itself is not particularly complicated - unless you hit a problem.
All sorts of things can go wrong, to give you a couple of real life examples:
Mitel changed the default value for a particular setting on T1/E1 links and didn't document it anywhere. On restoring our database the new software load mis-interpreted the value in our old database and ignored it as a safeguard - so it went to the default. This caused calls crossing these links to connect but they were garbled, caused a bit of head scratching to work out what was happening!
When upgrading a couple of our older systems the drives died. Bad sectors etc were present that were not affecting the system operation however the drives failed when they were asked to work hard repartitioning and installing the system software.
As an end user you can apply for access to MOL (Mitel On Line) which gives you access to the knowledge base and technical documentation, but not to download software. Even if you could download the software and wanted to do the upgrade you would need to licence your system to run the new version if the jump in version is sufficent, otherwise you will be unable to retrieve your current licences. Only a vendor can do this for you.