You can modify all aspects of an XMB Forum from the admin user's Control Panel. You can access this from the forum's address.
To modify an XMB Forum:
Go to the forum's URL. This will be something like http://yourdomain.com/forum/
Login using your admin username and password.
Click on the Control Panel link in the top left corner of your browser window. You can now modify the forum. Refer to http://www.xmbforum.com/ if you need more information about how to modify the forum.