Since the release of Ubuntu 10.04 LTS the update manager, allows for upgrading to 10.04.

Be aware ZevenOS 2.0 is not compatible with Ubuntu 10.04 LTS and shouldn’t be upgraded to this version!
ZevenOS 2.0 is based on Xubuntu 9.10 and the next version 3.0 will be shipped November 2010.
You can disable the upgrade notice in Synaptic packagemanager > Settings > Repositories > Updates.

(Thanks to rolan for this notice)