Upgrade from Ubuntu 20.04 LTS to Ubuntu 21.10

You can indeed only upgrade from one Ubuntu version to the next, or from one LTS release (Long Term Support release) to the next. 20.04 is a LTS. You can upgrade to 21.04, but you also will be able to upgrade to the next LTS, 22.04.

The system may not be customized too much in order to upgrade.

• Disable any PPA you added

• Fully update your system before attempting the upgrade:

  sudo apt update sudo apt upgrade sudo apt dist-upgrade

Then you can update with

sudo do-release-upgrade

If you do not get the update going, then the shortest pain will be a fresh install - overwriting your existing installation. Technically, that is by far the best, even though you then need to restore all your customizations.