Log in to your control panel.
Select the domain name that you want to apply the change to.
Click on HOSTING.
Click on NEXT.
Next to "Destination URL" change the url to the new target url.
Click on UPDATE.
You can also change the type of forwarding here. "Standard Forwarding" redirects a domain to a different URL with the destination URL showing up in the browser address window. "Frame Forwarding" redirects a domain to a different URL while the original domain name shows up in the browser address window. By default, forwarding account are setup as "Standard Forwarding".
There are several disadvantages to using "Frame Forwarding":

You will not be able to use a page title for your site.
None of the individual page titles will show up for your site.
The full url to pages will not show up in the address area of your browser.
You will not be able to give out links directly to pages without using the target url.