Actually, the change needs to be made on the OWA server, not the Exchange server. Exchange Server looks at the OWA server as little more than just another MAPI client, so that's why the previous poster's suggestion won't affect your problem. The only reason to tell Exchange the OWA server name is if you want non-MAPI clients to use OWA for the Outlook calender, meeting requests, etc. that are not normally available to non-MAPI clients. Exchange then mails meeting requests etc. with URL that opens an OWA calander in the client's browser. Pretty neat, but not what you need.
TechNet article Q243841 (http://support.microsoft.com/support/kb/articles/Q243/8/41.ASP?) explains how to change teh Exchange server in OWA. Doing a remove / reinstallwill also work but is probably more trouble than the registry edits mentioned in the above article.