1. Install 'update-manager-core'

    sudo apt-get install update-manager-core
  2. Execute 'do-release-upgrade'

    sudo do-release-upgrade

NOTES

  • If running from a ssh shell, it's highly recommended to run the upgrade from a 'screen' shell; that way you can resume the upgrade if ssh connection breaks
  • If 'do-release-upgrade' returns 'No new release found' when you know there's one, that's probably because you're trying to upgrade from a LTS (Long Term Support) Ubuntu version to a non-LTS one; to fix this, edit the file 'etc/update-manager/release-upgrades' and set the variable 'Prompt' to 'normal'; then rerun 'do-release-upgrade'