Hi there,
the Sites in the Library are designed for new builds.
They can be imported into an existing site BUT… you should first create a Staging or backup of your existing site to work from.
When you import the site , you must import the demo Content as this may contain GP Elements.
With that done, you would need to: Update Logos & Site Identity, Menus, Widgets and Copyright info as that would be imported from the site.
Then remove any pages you don’t require, i would suggest changing the post status of them ti Draft. As you may want to copy its content for use on updating your existing pages to match.