The best way to update the system is the above commands from console.
Just in case there are specific instructions to follow, limited to a specific time-frame update, we write them here:
https://forum.openmandriva.org/t/rome-major-upgrade-expected/4707.
Additional links, if any need for further explanation, are and will be provided in the same thread.
Hence you may want to have a look there before major upgrades.
Postedit:
No, you don’t want to switch Plasma5 to Plasma6 at this moment 
Even if someway doable it’s like looking for troubles.
For sure better to do a fresh install with Plasma6 environment. Final release will be published in a few days.