1. まず現在の Ubuntu 21.04 を最新状態にします。

$ sudo apt update 
$ sudo apt full-upgrade

2. 次に不要物を全消しします

$ sudo apt autoremove

3. 更新の設定をします。

/etc/update-manager/release-upgrades を開いて、Promptをnormalに設定します。

# Default behavior for the release upgrader.

[DEFAULT]
# Default prompting behavior, valid options:
#
#  never  - Never check for a new release.
#  normal - Check to see if a new release is available.  If more than one new
#           release is found, the release upgrader will attempt to upgrade to
#           the release that immediately succeeds the currently-running
#           release.
#  lts    - Check to see if a new LTS release is available.  The upgrader
#           will attempt to upgrade to the first LTS release available after
#           the currently-running one.  Note that this option should not be
#           used if the currently-running release is not itself an LTS
#           release, since in that case the upgrader won't be able to
#           determine if a newer release is available.
Prompt=normal

4. 最後に 21.10 へのアップグレードを実行します。

$ sudo do-release-upgrade

いつも通り数時間、時間がかかりますので、余裕をもって作業してください。

お疲れさまでした。