We have created a feature to change your main domain name automatically without contacting support. You can do so by logging into our billing control panel at https://billing.stablehost.com/clientarea.php.
Once you get logged in, click on 'My Services':
Find the plan you want to modify on the list, and click on it (just don't click on the domain name):
Click on the 'Change Domain Name' button under the Actions tab:
Place the new domain you want to use and click on "Change Domain".
You already have the domain as an Add-On domain.
If the domain you want to use as main domain is already added as an Add-On domain, then you have to remove the Add-On domain first, if you don't do this, you will get an error because you cannot set a new primary domain if that domain is already in use in our systems.
Back up your Add-On domain's public_html contents, as a safety measure.
In cPanel, remove the Add-On domain, this won't delete any public_html files.
Follow the instructions above to change the main domain.
Using your favorite file manager, remove the files in public_html and then move or upload the add-on domain's public_html files to your public_html root folder.