Upgrade Ubuntu 12.04 to Ubuntu 14.04

Ensure that your Ubuntu is up to date

sudo apt-get update && sudo apt-get upgrade -y

Install update manager core

sudo apt-get install update-manager-core -y

 Upgrade your Ubuntu

sudo do-release-upgrade -d

And follow next instruction carefully.....

Source:
http://www.namhuy.net/2853/upgrade-ubuntu-12-04-lts-ubuntu-14-04-lts.html