How to upgrade from Ubuntu 12.04 Precise Pangolin to 14.04 Trusty Tahr?

This tutorial shows you all the steps necessary to upgrade from 12.04 Precise Pangolin to 14.04 Trusty Tahr. Important note: As a prerequisite you have to configure the Software & Updates manager properly. Launch the Software & Updates manager and choose the Updates tab. From the option "Notify me of a new Ubuntu version" select "For long-term support versions". If you select "For any new version", you will be upgrading to Ubuntu 12.10, if you select "never" you will not see the upgrade at all.