The first time you run the Zimbra installation you are aksed if you want to set the domain name, the default is to use the FQDN of the server, if you answered yes then you could have done it in there. The only method to go through that procedure is to uninstall Zimbra the reinstall to get to the prompt.
If you wish to rename a working server then back it up and the run
zmsetservername, it should be straightforward but make sure you read and understand the document before running it.
You should also make sure you understand the implications of having a correct DNS set-up, as mentioned in the article.