You can change the primary domain name associated with your A2 Hosting account. Before you do this, please make sure that the new domain is not currently set up as an addon or parked domain. To avoid downtime, make sure the new domain is ready to work with the server before requesting the change. See this article for details.
When you are ready to switch your account to the new domain, open a ticket on the Customer Portal at https://my.a2hosting.com and we will make the change.