You can easily change the primary domain name for your hosting account. In order to do so , you should go to your SiteGround Customer Area > Manage (next to the account whose primary domain you wish to change) > Primary domain name change > Go.

You can also change your primary domain from your Customer's area > Get support > Report a hosting related issue > Domain name change.

You will be forwarded to a form which will guide you through the automatic process of changing the primary domain name. As soon as you have read the instructions, you can proceed by clicking I agree to proceed with the domain name change. You will be prompted to fill in the new domain name  you chose and when ready, will be able to complete the process by clicking on the Proceed with the domain name change button. Please make sure you have typed in your new domain name correctly before submitting it, so that you do not need to go through this process again.

Please note that if the new domain name is not registered, you will need to register it first. You can do so from your Customer Area, Order Extras section, New Domain.