The Pocket Site

To upgrade in place for releases that are no longer officially supported please see…
http://www.nzlinux.com/2009/06/upgrading-older-releases-of-ubuntu-when-support-ends/

Depending on the installation it might be substantially faster to do a clean reinstall and migrate your content from backups. YMMV.

If you are already on one of the LTS releases then there should be a safe upgrade path available per https://help.ubuntu.com/community/UpgradeNotes