Upgrade Ubuntu 12.04 to 14.04.5 lts | Easy Step by Step Tutorial How to Upgrade Ubuntu (Linux)
Watch how to upgrade Ubuntu 12.04 to 14.04.5 lts Trusty Tahr
An easy step by step tutorial about How to upgrade Ubuntu Linux.
First of check that you are fully up-to-date.
Double check by opening the Update Manager application from the dash and installing all updates listed.
Hit the return/enter key and, if prompted, enter your user password.