Linux Poison shows you how to upgrade Ubuntu using an alternative ISO image

If your computer is not able to run the standard Desktop installation CD, you can use an Alternate installation CD instead. The Alternate CD also allows more advanced installation options which are not available with the Standard LiveCD.

To upgrade Ubuntu from an ISO file, you need the Alternate version CD, NOT the regular Ubuntu Desktop CD. Alternate CD ISO image file of any release can be found here: http://releases.ubuntu.com/.

  How to Upgrade Ubuntu Using an Alternate ISO Image