That is, if you haven’t got a setting that only lets you upgrade to LTS (Long Term Support) releases – which if you’re running Ubuntu 12.04 (Precise Pangolin), you probably do.
Solution: open the Software Centre, then go to Edit… Software Sources. On the Updates tab change “Notify me of a new Ubuntu version” to “For and version” and close it all down. Run
update-manager -d and it should be all easy from there!