Upgrade old Ubuntu

If it's really old and not supported anymore, you'll need to first change the repositories in /etc/sources.lst:
$ 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 and upgrade:
$ sudo apt-get update; sudo apt-get upgrade

Then do the release upgrade:
$ sudo do-release-upgrade

References:
https://askubuntu.com/questions/779189/how-to-upgrade-a-server-from-12-04-to-16-04
https://askubuntu.com/questions/91815/how-to-install-software-or-upgrade-from-an-old-unsupported-release