You certainly can!

Simply go to Administration/Virtual domains, click on the 'Properties' icon (the hand) corresponding to the domain name.

Note: this will not change the domain name in email identities (signatures and the 'From' address) or in the users' personal details.

You may want to set up a domain alias so that users will still receive emails sent to the old domain address. In Administration/Global aliases, create this alias:
Alias: *@olddomain.com
Target: *@newdomain.com

In this way, if someone sends an email to the old address, it will still be received.