It’s done using a command line utility called “do-release-upgrade”, it’s part of the update-manager-core package, so all you have to do is the following:
“sudo apt-get install update-manager-core”
“sudo do-release-upgrade”
That’s it, simply!
Source: https://help.ubuntu.com/12.04/serverguide/installing-upgrading.html
Just Some Techie Notes! 