Unfortunately, it is not possible to rename a domain. You can only change/replace it with another domain name.

SiteGround customers can do this following the instructions below:

1. Order a new domain. This can be done from your Customer's area -> the Order tab.

2. Change the primary domain name of your account. This can be done from your Customer's area > Manage (next to the hosting account whose primary domain you wish to change) >  Primary domain name change.

You can also do this from your HelpDesk > Domain name change.