If u already have Ubuntu 7.10 installed on your machine, here is the way how to upgrade to 8.04.

1. Press Alt-F2 and type update-manager --devel-release
2. Click the Check button to check for new updates.
3. A message will appear informing you of the availability of the new release.

4. Click Upgrade.
5. Follow the on-screen instructions.

U are done!