If you'd like to use WordPress instead of Hostinger Website Builder, you simply need to delete the website built with the builder in order to release your domain name.
Before deleting a website created with Hostinger Website Builder, you can export website files
A deleted website cannot be restored
In hPanel → Websites, delete the website that you've created:
Create a new website and choose WordPress as your preferred website-building platform:
Click Create or migrate a website
Select Build a website
To finish the setup, create a WordPress account, choose the template you like (you can customize it later), or skip it and insert the domain name that you've previously released by deleting the website built with Hostinger Website Builder.