As your domain name has already been purchased and registered, it is not possible to change the name. The only way to do this is to buy a new domain. Here's how to do this:
1. Go to Settings > Publishing.
2. Click the Unpublish Site link. A dialog box will open up asking to confirm that you want to unpublish.
Type "Unpublish" into the text box provided and click Unpublish site.
Your site is now unpublished.
To republish your site:
1. Click the Publish button.
2. Select Search for a custom domain, choose the new domain you would like to purchase, and click Next.
3. Follow the steps to purchase your new domain and, once completed, your site will be published to your new domain.
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