That’s because most probably your theme’s index.php is including somewhere the content of the template via a function (i.e. get_template_part()).
Also, all your page’s contents are (and should be) stored in the database and displayed via queries; the contents are not (and they shouldn’t be) hard-coded in any file.
Usually when you want to edit a WordPress page you do it from the admin panel. Open your Dashboard, from the left menu click on “Pages”, then click on “All pages”, select the page you want to edit (from your website’s top menu I’m guessing it’s called “Home”, maybe?), and click on “Edit”. You should see the contents there.