Background
A long (in computer terms) while ago I bought an Intel NUC as my first venture into building a computer from scratch (not exactly challenging - I only had to install the memory and the hard drive and then boot it off of an Ubuntu Live USB).It is not particularly fast (still faster than any of my Raspberry Pis), but it is useful for some tasks (amongst others it runs Inkscape and does not get tied in a knot when logging into YouTube). It gets hauled out occasionally when I need it.
I dutifully update the operating system (starting with 14.04 Trusty Tahr). Now Trusty Tahr is a Long Term Support release but this is not a production machine so with each release version I updated it to the latest stable version.
And then I did not use it for a while.
I recently needed to upload something to YouTube, and once completed I went to update the operating system...
At which point the updater could not find the repository as Zesty Zapus was retired on the 13th of January 2018.
First point of call was a Google search for information. This brought me to reference 1. This was informative, but did assume that the Aptitude package was already installed.
A further Google brought me to Reference 2 for a simpler command line method.
Upgrade method
Please ensure you have backed up the machine before upgrading.
These instructions are supplied "as is" and with no guarantees.
Please use them in conjunction with the references and common sense.So here is my version, based on the two references and my experience (please read the referenced pages for more detail).
First the sources.list file needs to be edited - my installation was fairly vanilla so had no strange sources.
Either copy the existing file or comment out the existing entries.
Command line:
sudo nano /etc/apt/sources.list
Replace with the following lines (replace the "zesty" with your version if that is different):
## EOL upgrade sources.list
# Required
deb http://old-releases.ubuntu.com/ubuntu/ zesty main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ zesty-updates main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ zesty-security main restricted universe multiverse
The next stage is to run the apt update command to obtain the last version information (using the new repositories):
sudo apt update
Once that has completed the versions of the software needs to be upgraded.
sudo apt upgrade
This will take some time.
Once this has completed you need to run the distribution upgrade.
sudo apt dist-upgrade
If this does not work (reporting no suitable upgrades), restart the computer.
Once it has completed you need to restart the computer again.
After you have logged in again, you run the release upgrade. This will take a long time, hours.
sudo do-release-upgrade
Once that completes you will have an installation of Artful Aardvark (17.10). This is only supported until July 2018 so you will need to upgrade it to Bionic Beaver.
This should be automatic, except you have slightly messed up the settings by editing the sources.list file.
Bring up the settings panel and select the updates pane. Select the sources by clicking in the tick boxes. It will then ask you if you want to check if there are updates. If it does not, try swapping the check interval (never to daily seems to work).
You can then follow the instructions for the upgrade to Bionic Beaver.
References:
https://help.ubuntu.com/community/EOLUpgradeshttps://ubuntuforums.org/showthread.php?t=2382832
https://en.wikipedia.org/wiki/Ubuntu_(operating_system)#Releases
