It's not currently possible to change the master domain.
We recommend leaving the domain and instead adding a new name as a parked name. This will give the same outcome and will not use the recorded main domain, you will be able to upload files to public_html as normal and also create email addresses.
If you would prefer it to be changed, we are able to remove the hosting package from our servers and then re-create using the domain you wish to use.
Please raise a ticket and request that your account be re-created with the new domain.