I do everything myself as an end user. Normally it takes carefull planing, reading ALL release notes and available KB, obtaining all the licenses, making backups (I make one before I proceed and one during the process), creating plan B and plan C. Due the fact that some of releases will upgrade boot loaders it is possible that system will not boot after the procedure, so it would be nice to have a spare motherboard or bare hardware controller handy.
Normally your Provider will have all this said above and have done the procedure at least in the lab environment and at least once. Plus they have an option to contact Mitel support directly and resolve complex problems right on the spot.
Now it is up to you to measure risks to destroy your business communication system for unpredictable amount of time or let your VAR to deal with all problems within scheduled downtime.