If you want to continue using an outdated release then edit /etc/apt/sources.list and change archive.ubuntu.com and security.ubuntu.com to old-releases.ubuntu.com.

You can do this with sed:

sudo sed -i -re 's/([a-z]{2}\.)?archive.ubuntu.com|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list

then update with:

sudo apt-get update && sudo apt-get dist-upgrade

Sometimes, it might be faster to create backups of your system and reinstall using supported release instead.