Hi there,
installing a site from the Site Library will NOT remove any of your posts or other content such as images etc.
However we don’t generally recommend installing a library site on a live site, as you will need to make ‘changes’ such as updating the logos, menus, widgets etc that the site will import.
If you want to try we recommend moving your site to a Staging Site, so you can do the install, make any changes without affecting your live site. Most hosts provide an option to create a staging server for you.