This article will help you to resolve any issues relating to the domain name used in your sitemap.
Scroll down to the Create your website section and click on the Website Builder icon.
Click on the Login button next to the relevant package.
Click on the Manage link from the top menu.
Select Preferred Domain from the left hand side menu.
Select the required domain name from the drop down menu and then click on the Save button.
Click on the Website link from the top menu and Publish your website again to update your sitemap.