Probably a dumb question, but is this command executed on the host terminal? No UI needed? Thanks
Call up a terminal / console of your choice on your computer - not the Openmediavault computer.
Enter the ssh IP number of the Openmediavault computer (e.g. 192.168.178.52) and log in with your login data.
Then enter "sudo omv-release-upgrade" in the console and confirm with Enter.
Now enter the admin password again and the whole thing should be installed.
It may take a while, depending on how fast your system is.
If you want to be on the safe side, make a backup copy of your system beforehand - not the data.