I tried
sudo apt update
And got the error:
$ sudo apt update
Ign:1 http://ppa.launchpad.net/jonathonf/python-3.6/ubuntu yakkety InRelease
Hit:2 http://archive.canonical.com/ubuntu yakkety InRelease
Ign:3 http://gb.archive.ubuntu.com/ubuntu yakkety InRelease
Err:4 http://ppa.launchpad.net/jonathonf/python-3.6/ubuntu yakkety Release
404 Not Found
Ign:5 http://security.ubuntu.com/ubuntu yakkety-security InRelease
Ign:6 http://gb.archive.ubuntu.com/ubuntu yakkety-updates InRelease
Ign:7 http://gb.archive.ubuntu.com/ubuntu yakkety-backports InRelease
Err:8 http://security.ubuntu.com/ubuntu yakkety-security Release
404 Not Found [IP: 91.189.91.26 80]
Ign:9 http://gb.archive.ubuntu.com/ubuntu yakkety-proposed InRelease
Err:10 http://gb.archive.ubuntu.com/ubuntu yakkety Release
404 Not Found [IP: 91.189.88.161 80]
Err:11 http://gb.archive.ubuntu.com/ubuntu yakkety-updates Release
404 Not Found [IP: 91.189.88.161 80]
Err:12 http://gb.archive.ubuntu.com/ubuntu yakkety-backports Release
404 Not Found [IP: 91.189.88.161 80]
Err:13 http://gb.archive.ubuntu.com/ubuntu yakkety-proposed Release
404 Not Found [IP: 91.189.88.161 80]
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/jonathonf/python-3.6/ubuntu yakkety Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://security.ubuntu.com/ubuntu yakkety-security Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://gb.archive.ubuntu.com/ubuntu yakkety Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://gb.archive.ubuntu.com/ubuntu yakkety-updates Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://gb.archive.ubuntu.com/ubuntu yakkety-backports Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://gb.archive.ubuntu.com/ubuntu yakkety-proposed Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
So, I tried this:
Getting following error after the commad sudo apt-get update on Ubuntu 16.04 - Stack Overflow but it didn't work. Then I tried Multiple issues with "sudo apt-get update" - Ubuntu Forums
Which now means I get this, when I try a dist upgrade from 16 to 18, via the Software Updater tool (silver circle icon): Getting following error after the commad sudo apt-get update on Ubuntu 16.04 - Stack Overflow