From Server -> IP Addresses
Click the number below the Hosting column. This will take you to a screen showing the domains associated to that IP address. Click the radio button to the left of the domain you want to be the default and then click the link that says "Set as Default".
You can also use the websrvmng tool from the command line. Here are the options relating to the default domain name:
--set-default-domain --ip-address=<ip_address> --vhost-name=<domain_name>
sets default domain for IP address. Thus, all HTTP requests which have come on given IP and for which there is no appropriate domain, should come on given default domain
--reset-default-domain --ip-address=<ip_address>
delete default domain for IP address