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.
First, back up this file:
sudo cp /etc/apt/sources.list{,.bak} # Older versions
sudo cp /etc/apt/sources.list.d/ubuntu.sources{,.bak} # Newer Ubuntu uses this file instead
In those files, replace "archive" and "security" in the URLs with "old-releases":
sudo sed -i -re 's/([a-z]{2}\.)?archive.ubuntu.com|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list{,.d/ubuntu.sources}
Finally, upgrade to a newer release:
sudo apt-get update && sudo apt-get dist-upgrade