Long story short, you can't... at least not thru the interface. Instead, you need to totally blow out your configuration and reconfigure the server.
However, it's not as bad as it seems - there's not so much to configure that it should be too big of a hassle.
If you have console access, close your databases, and make note of your settings.
- Stop the Filemaker Service.
- Assuming a default installation, go to c:program filesfilemakerfilemaker serveradminconf and delete the 4 XML files in there.
- Restart the Filemaker Service
- Reconnect to the Filemaker Console
It'll re-walk you thru the wizard to set up the server, and the first thing you do is set up a username and password. Your databases (again, assuming a default installation) will already be there, ready to go...