An upgrade is the process of going from an earlier version of Ubuntu to a newer version of Ubuntu. An example of this would be going from Ubuntu 6.06 (Dapper Drake) to Ubuntu 6.10 (Edgy Eft). Skipping versions is not advised and can cause a lot of damage to your installation. If you are using an earlier version, and want to 'skip' a version, the only safe way is to backup your data and do a fresh install, or to progressively upgrade to each successive version. An example of this would be if your are running 6.06 (Dapper) and want to upgrade to 7.04 (Feisty). You would have to first upgrade to 6.10 (Edgy), and then again to 7.04 (Feisty) or simply do a clean/fresh install of 7.04 (Feisty). An easy way to test a newer version of Ubuntu on your machine for compatibility is trying the Desktop CD (Live CD) before upgrading.
https://help.ubuntu.com/community/Insta ... OldVersion