1. Go to site settings (top right, yellow button).
2. Select "Site Address - Domain Name".
3. Select the first green button "Purchase your own domain name with YorName.com".
4. Use the button "Go to YorName.com" and purchase your new domain name.
5. A couple of hours after the purchase, publish your site to link it to your new domain name.
Watch the tutorial video:
How To Register A Domain Name