In Unbounce, when you’re ready to publish the page, just make the URL the same as your root WordPress URL. In other words, instead of using “mysite.com/page-name” just leave it as “mysite.com” and it should publish to your homepage. It will override any existing homepage you have already set up on WordPress with the Unbounce page.
However, it won’t delete that page. So that way, you can always unpublish your Unbounce page and the old WordPress homepage should show up again.