You cannot currently upgrade directly from 12.04 to 16.04; you have to upgrade to 14.04 in-between. So simply running the following will upgrade you to 14.04:
Then if you don't want to upgrade again to 16.04, just stop there rather than performing another upgrade.
Or (for the GUI way - the relevant program is also known as the Software Updater and can be searched and found in the GUI under that name):
For this though once you are completely up-to-date with all the latest updates it will show you an Upgrade... button which will allow you to upgrade to 14.04.