You can change the primary domain name for your hosting account to another one by using the Domain Name change feature in the HelpDesk. Goto your:

Customer Area --> Get Support --> Report a hosting related issue --> Domain name change

The new domain name should meet the following requirements in order for a successful domain name change:

1. The domain name should not be parked to the account.

2. The domain name should not be applied as an addon domain for the account.

3. The domain name should be pointed to the correct name servers for your account.

4. The domain name should not be applied as primary for another SiteGround hosting account.