Don't quite understand, what problems are you having with the .com domain? Is it taken? Is it yours? Is it playing up with Directadmin? Give us a better clue of whats going on
If you purchased a hosting account from a company and host a domain by them. In order to rename it, you will need first to register a domain by a domain registrar. If your hosting company (re-)sells domains, you might ask them directly. These forums are community based, you might find a link to it in a hosting panel Directadmin, which is likely used by your hosting company.