When signing up with us, you will be asked to choose a primary/main domain for your hosting service.
If later on you would like to change that domain to another one (i.e. add a new website), that can easily be done.
However, keep in mind that you cannot change the primary domain to one which is already hosted in your account (i.e. an existing site).
Meaning that the new primary domain will have to be a completely new site.
In order to change the primary domain, first off you need to go to your hosting panel, which can be done via our website -> My Services -> WordPress hosting -> Manage all sites in this account.
Note that the last step of choosing "Manage all sites in this account" is not shown on the screenshot, as it is in another page.

Once in the hosting panel, choose the "Websites/SSL" section.

In this section you will be able to see all of the websites that are currently hosted in your account.
The primary website would be the first one. On the right side of the domain name, choose the "Settings" option.

Once chosen, you'll see a pop-up and inside you will find the option to change the domain.

All you have to do is fill in the new domain in the text field and then choose "Save changes".
If you run into trouble and get stuck, please raise a support ticket here:
https://wpxhosting.com/tickets/new/
and we will reply ASAP.