under a ubunto server with plesk panel 11.0.9 running I have installed Atmail webmail servive. But now we want to change to Horde, so I want to uninstall atmail but can't find hoe to do it under plesk.
You don't actually need to uninstall one webmail to use another. Just make sure the webmail you are going to use is installed and then switch to it (as default one) in server mail settings. IIRC, clients can also choose webmail for subscription, if several are available.
Not 100% sure this still works with Plesk 11, but I guess it should. Anyway, this will just unregister AtMail as a webmail. It will not be removed from the system per se, only from the panel UI.