UPDATE: You can now create database backups on the Update Centre screen but we still recommend using either the method below, or the tools in your web hosting account, and create a total of two backups based on different methods
In our documentation, and on the Upgrade page itself, we emphasize the importance of backing up your database before upgrading to a new version. There a number of things that can go wrong during a sensitive operation like this, many not even related to the application itself, for example; you could loose your Internet connection at the precise moment you click the button, or your web server could experience technical problems beyond your control or knowledge at the exact same time you executing an upgrade.
During the upgrade process, our logic creates a backup of your entire MOHOBOOK folder, but it does not attempt to backup your database for the same reasons mentioned above. We hope you, our customers, realize the importance of having a reliable backup of your database. It could literally save you a world of aggravation in the event of an unforeseen problem.