In the example below, you can see that the default subdomain 2488192929.onlinehome.us is linked to the Click & Build Joomla! website named AccountHelp (A). After clicking the domain that is going to be linked to Joomla! (B), click Edit Destination (C).
1&1 Domain Center
In the Destination section, choose Webspace. Next, expand the views of your available folders to select the target directory, which is your Joomla! root directory (See also Step 3 (A).
Webspace Assignment Screen
Edit Destination Confirmation
Wait until the domain's Target details show the updated directory.
If you just registered the domain, this can take up to 24 hours.
Now it is necessary to remove the pointer to the old domain in Joomla! itself in order to assign the new domain. To do so, edit the file configuration.php in the main Joomla! directory via FTP or Webspace Explorer.
While editing the configuration.php, add the entry: public $mosConfig_live_site = 'http://www.1and1help.com'; (enter the actual domain name instead of "1and1help.com").
Save the file and upload it back to the Joomla! root directory.
Your Joomla! Website is now connected with the new domain. The manual change to the new domain name will not be reflected in the 1&1 App Center. Here, the original domain will still be displayed.
1&1 App Center
For additional information, you may want to reference: