How to install software or upgrade from an old unsupported release?

The repositories for older releases that are not supported (like 11.04, 11.10 and 13.04) get moved to an archive server. There are repositories available at http://old-releases.ubuntu.com
The reason for this is that it is now out of support and no longer receiving updates and security patches.
I would urge you to consider a supported distribution. If your computer is too old in terms of memory or processor then you should consider a distribution such as Lubuntu or Xubuntu.
If you want to continue using an outdated release then edit /etc/apt/sources.list and change archive.ubuntu.com to old-releases.ubuntu.com
You can do this with sed

sudo sed -i -e 's/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.
source

---

To upgrade to a new release:

Once you have performed the above steps to switch to the old-releases mirrors, update the Update Manager and then do do-release-upgrade:

sudo apt-get update
sudo apt-get install update-manager-core
sudo do-release-upgrade

Source : http://askubuntu.com/questions/91815/how-to-install-software-or-upgrade-from-an-old-unsupported-release