Hi there,
The Site Library was created for new installs.
But, you can import a site on a pre-made site. Things to note:
1. It will NOT remove any of your existing content. That will remain in place.
2. It will add new pages, posts and elements from the demo content, which you will need to remove if not required.
3. It will replace theme elements such as Logo, Navigation and Widgets ( if used ) that will need swapping back.
We therefore recommend that you move your site to a staging server ( your host should be able to help with that ) and then import the site from the library, and then make your fixes. This way you can do it without harming your live site. And when you’re happy with your changes, your host can push the site live.
If your host doesn’t provide a Staging server option then at the very least make a back up of your site before proceeding, just to be safe 🙂