Back in the early days of Linux, the changes to the operating system were so dramatic that even when updating withing the same distro, in-place point release updates were problematic enough and it was always recommended that a major version release be done from scratch. There were just too many major changes to the underlying subsystems and package naming conventions. Doing an in-place update was just asking for trouble.
Modern distros have matured quite a bit even though there are occasionally some major subsystem changes being made. Yet it's never been easier to do an in-place major version upgrade.
I just did the upgrade on several Mageia5 systems and the results were consistent and satisfactory, not to mention simple.
Always backup critical information. Have a Mageia5 recovery disc on hand.
Read the Release Notes, the Documentation and the Errata for Mageia 6. Other than updated applications, the biggest changes are support for UEFI and the move to GRUB2. If you are upgrading from a GRUB/Mageia5, you will keep GRUB.
You can always download an ISO image to do the upgrade rather than do an in-place installation if your Internet connection is slow. DO NOT attempt an upgrade from the Live ISO image as it will overwrite your existing system.
Upgrade From GUI
Open a terminal window. $ sudo mgaapplet-upgrade-helper --new_distro_version=6
And you're done. This essentially does all the following individual commands.