1. Log into the Customer Control Panel with your main account's username and password.
2. Click on the "Websites" tab.
3. Click on your domain.
4. Click on the "File Manager" link.
5. You'll see all the files where you domain name is mapped to and you can manage the files through there.
Note: The other option is to use and FTP program like FileZilla (https://filezilla-project.org/) and mange the files for your site that way.