There is no setting in your name.com account to change the domain for the hosting, but we can manually change it on your behalf - per request. In order to change the domain for the hosting, we would first need you to backup any content related to the domains you want to switch FROM and TO. A guide on how to create backups can be found here. We suggest you make the backup because the structure will get shuffled around and to keep you on the safe side.
If you have Namebuilder or Unbelievable hosting and the domain you want to change TO is currently setup as an Addon domain, then it needs to be removed from the hosting before we can make the switch. To remove the domain, simply log in to your hosting cPanel, click the Addon Domains icon and proceed to remove it.
Once you have saved/backed up for the domains in question (and removed the potential addon domain) then please submit a ticket and clarify what domain you want to change the default domain FROM and TO. To give us some context please also include a short description that you wish to change the domain for the hosting, prepped the domain for the setup and that you have read this knowledge base article.
Note: If you have Wordpress, it's not as simple as changing the primary domain for the hosting. You would also need to change it in the Wordpress settings and configuration as well. We cannot change the Wordpress domain for you but there are plugins to make this change easier for you and you can also read their help tutorial here.