How do I upgrade an end-of-life Ubuntu distribution?

My do-release-upgrade fails. I get the following error message:

Checking for a new Ubuntu release
Your Ubuntu release is not supported anymore.
For upgrade information, please visit:
http://www.ubuntu.com/releaseendoflife

Err Upgrade tool signature
  404  Not Found [IP: 2001:67c:1360:8c01::19 80]
Err Upgrade tool
  404  Not Found [IP: 2001:67c:1360:8c01::19 80]
Fetched 0 B in 0s (0 B/s)
WARNING:root:file 'raring.tar.gz.gpg' missing
Failed to fetch
Fetching the upgrade failed. There may be a network problem.

So it says my version is not supported anymore. I have Ubuntu Quantal (12.10). What should I do now?

I had this exact same issue and this answer solved the issue for me.

In summary:

The repositories for older releases that are not supported get moved to an archive server. There are repositories available at http://old-releases.ubuntu.com

So you have to rewrite your /etc/apt/sources.list file in order to use this server. Use the following sed command to do this:

sudo sed -i -e 's/archive.ubuntu.com\|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list

Also comment out any entries in sources.list that point to mx.* servers

Now you are ready to do your update:

sudo apt-get update
sudo apt-get install update-manager-core
sudo do-release-upgrade

You have to change all apt-sources from archive.ubuntu.com or security.ubuntu.com to "old-releases.ubuntu.com".

If you use the standard hostnames, as described above, you an replace this using:

sudo sed -i -e 's/archive.ubuntu.com\|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list

If you are using subdomains of mirrors in your country (e.g. "us.archive.ubuntu.com") use this command:

sudo sed -i -e 's/us.archive.ubuntu.com\|us.security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list

old-releases.ubuntu.com doesn't support country-code-subdomains.

Change all of your apt-sources to "old-releases.ubuntu.com". This sed command works with any/all ubuntu.com URLs and saves a backup copy in case you need to revert:

sudo sed -i.save -e 's/\/\/.*ubuntu.com/\/\/old-releases.ubuntu.com/g' /etc/apt/sources.list

Upgrade all packages in your current (old and outdated) release and make sure you have the update manager installed:

sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
sudo apt-get install update-manager-core

Reboot:

sudo reboot

Upgrade to the latest release:

sudo do-release-upgrade