Go to Software and Updates (in your control panel or system area, depending on your flavor). Go to the tap for “updates” and set the “notify me of a new Ubuntu version” to “For any new version.”

(See picture above.)

Then, in either a terminal or in the box you get when you hit Alt-F4, type the following and hit enter:

update-manager -cd

You might get something that looks like this, and you can hit the upgrade button and follow instructions. Good luck. Have a backup. Should work fine.

If things don’t work fine, try THIS.