18 months of support for Ubuntu 7.04 will expire on Sunday October 19th, 2008. Security updates will no longer be provided for Ubuntu 7.04.
Instructions and caveats for the upgrade to 7.10 may be found at https://help.ubuntu.com/community/GutsyUpgrades. Note that upgrades to version 8.04 LTS and beyond are only supported in multiple steps, via an upgrade first to 7.10, then to 8.04 LTS.