To make apt-get update work, we simply need to edit /etc/apt/sources.list and update all URLs from security.ubuntu.com and archive.ubuntu.com to point to old-releases.ubuntu.com, as per the following sed command:

sudo sed -i.bak -r 's/(archive|security).ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list

After this, sudo apt-get update seems to run without a hitch. Of course if you do this, you’re accepting to use an insecure system, and should do sudo apt-get install update-manager-core; do-release-upgrade as soon as possible.

https://gist.github.com/dergachev/f5da514802fcbbb441a1