I tried a fresh install from my old 20.04 desktop to see if I could get Musescore running better. I thought I'd try to upgrade to more current version of the OS before installing my software, but I can't seem to figure out how this time. An OS upgrade doesn't appear in Ubuntu software or Ubuntu software/updates. Updates says everything is up to date, but it's still 20.04. I thought the first time I installed, the system kept bugging me to upgrade the OS. Did I turn those notifications off this time? Is there an OS upgrade procedure I can try?