Press the Superkey (Windows key) to launch the Dash and search for Update Manager
Check for updates
Select the tab called "Updates". Then set the "Notify me of a new Ubuntu version" dropdown menu to "For any new version". Press Alt+F2 and type in "update-manager" (without the quotes) into the command box. The Update Manager should open up and tell you: New distribution release '15.04' is available. Click Upgrade and follow the on-screen instructions.
Install the upgrade
A message will appear informing you of the availability of the new release. Click Upgrade and follow the on-screen instructions.
If you need some help installing Ubuntu, please check out our step-by-step guide.