How do I disassociate an existing domain in order to point it to a new website?

Hi,
I'm trying to point my domain to a revised website but I get the following error: "Domain can not be associated to the selected site.
The selected site already has a domain associated to it. WebsiteBuilder supports only 1 domain association for a site at a time. Please disassociate the existing domain and try again."
Couldn't figure out how to do that and didn't find anything in the help database that worked. I'd appreciate any guidance.
Thanks