I have checked through Internet Forums, and I found some so-called step-by-step guides for upgrading Magento CE. It seems that there are various methods each one with its pros and cons. Some people say to use Magento Connect Manager (MCM), others say to do the upgrade via command line, and others say to install the new version as a new website on the same server, transfer modules from the old to the new version and finally migrate the content and database.
I have upgraded Magento CE 1.5 to 1.7.02 on my own in the past, and it was not an easy and straightforward process, as the process was somehow successful on my test environment, but I had surprises on the production environment. I could say a lot here, but I'd just like to underline that MCM is definitely not appropriate for Magento CE upgrade, while UNIX / PHP / MySQL deep knowledge is absolutely a must.
Is there any official guide for upgrading Magento CE issued by Magento? I mean some document addressed to IT professionals including a checklist of prerequisites, step-by-step actions, acceptance testing, troubleshooting.
If you are looking for an official documentation , please visit Upgrading to and Verifying Magento Community Edition (CE) and Enterprise Edition (EE), Part 1 by Magento.
1) Create a copy of the site to be upgraded, including the database.
2) Upgrade the development copy from step 1, making note of all the errors and subsequent fixes.
3) Take the list of errors/fixes from step 2, and use that to upgrade the live version of the site.
4) Hopefully all the fixes worked and now your live site is upgraded.
On the other hand, if you have many problem, errors in upgrade magento, I think you can use Magento upgrade extension. However, it seems to be pay some cost.