How to update Rockstor is described here:
https://rockstor.com/docs/installation/install.html#install-updates-from-the-web-ui
Independant of the Rockstor Version, the underlying OS also has also a Version (openSUSE Leap 15.4, 15.5., 15.6). How to upgrade the distribution is documented here:
https://rockstor.com/docs/howtos/15-3_to_15-4.html
Regarding the paid licence - only the stable updated do need a licence. You can activate the testing update channel without a licence:
https://rockstor.com/docs/interface/system/update_channels.html#update-channels
And finally, if you would rather install a new Version, you can also export your configuration, re-install Rockstor and then import the config again.
https://rockstor.com/docs/howtos/reinstall.html
The guide is a bit old and the screenshots don’t match the current version exactly, but the overall procedure is still the same.
Be aware though, that some of the rockstor configuration is not imported properly at the moment: