~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 16.04.6 LTS
Release:        16.04
Codename:       xenial

¹⁾

~$ cat /etc/os-release
NAME="Ubuntu"
VERSION="16.04.6 LTS (Xenial Xerus)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 16.04.6 LTS"
VERSION_ID="16.04"
HOME_URL="http://www.ubuntu.com/"
SUPPORT_URL="http://help.ubuntu.com/"
BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/"
VERSION_CODENAME=xenial
UBUNTU_CODENAME=xenial

~$ sudo apt update && sudo apt upgrade
.........

~$ sudo apt install update-manager-core

今回は以下のようにインストール済みと表示された

update-manager-core is already the newest version

/etc/update-manager/release-upgrades

# 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=lts

LTSが指定されている

Prompt=lts

~$ sudo do-release-upgrade
Do you want to start the upgrade?


3 installed packages are no longer supported by Canonical. You can
still get support from the community.

3 packages are going to be removed. 86 new packages are going to be
installed. 395 packages are going to be upgraded.

You have to download a total of 137 M. This download will take about
40 seconds with your connection.

Installing the upgrade can take several hours. Once the download has
finished, the process cannot be canceled.

 Continue [yN]  Details [d]

「y」で実行

今回は「keep the local install version xurrently installed」を選択

Searching for obsolete software
Reading state information... Done

Remove obsolete packages?


4 packages are going to be removed.

 Continue [yN]  Details [d]

「y」で実行

System upgrade is complete.

Restart required

To finish the upgrade, a restart is required.
If you select 'y' the system will be restarted.

Continue [yN]

WSLで、rebootできないため「N」

~$ exit

~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.4 LTS
Release:        18.04
Codename:       bionic