Because Ubuntu 12.04 is a Long Term Support release, meaning that it is supported with bug fixes and security updates for 5 years, you won’t find a popup telling you that a new version is available to install, at least not yet. Canonical will not start notify LTS desktop users of an update until July, when the first point release (14.04.1) is set to go live.
Upgrade Ubuntu 12.04 LTS to Ubuntu 14.04 LTS
Open the Dash and open the Terminal application. When the application opens click inside it and type the following command carefully:
sudo update-manager -d
Hit the return/enter key and, if prompted, enter your user password.
The Update Manager application will open after a few seconds with a prompt to upgrade. Clicking this will begin the process.