could not update from 7.10 to 8.04

I wanted to update my xubuntu 7.10 to 8.04. I went to xubuntu's web site and got instructions how to do it. The site said that I should open my update manager and get the latest updates for 7.10, then click on the "check" button again and the update manager would offer the upgrade. I did everything as instructed, but no offers for upgrade happened when I clicked the "check" button. I rebooted my pc but opened the update manager again. It said that the system is up-to-date but the option for the upgrade is still not there.

Any help? Thanks.