apt-get upgrade getting: Err http://security.ubuntu.com raring-security/universe amd64 Packages 404 Not Found [IP: 80]

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.