When using Hostinger Website Builder, you can easily change the domain name.
NOTE: By changing the domain, you'll lose all email data related to that domain, and your email plan will be reset
In hPanel, go to Websites, and find the website in question. Then, click on the ellipsis ⋯ button and select Change domain:
Confirm that you understand that all email data related to the domain will be deleted and click Change.
Finally, choose the preferred domain to connect, and click Change again.
After changing the domain, your website will be up and running within 24 hours.