OK, so some things are template drive like your:
Blog and Archive pages
Single Post Heros
So when you import a site from the library you will see those things change.
But the Content inside a page will will only change for things like Fonts and Colors and general spacing.
So if for example you imported the Stream site:
https://gpsites.co/stream/
You won’t see your static pages change to how that home page would look.
You would need to edit those pages instead.
Does your host offer a staging site option ?
As this would allow you to create a staging copy of your website, where you can import a site from our library, make the changes that will be required without affecting your live site.
Then when you’re happy the host can push the stating site to the live site.