The main navigation menu is located in header.php. You'll see a section similar to the following.
<nav id="site-navigation" class="main-navigation" role="navigation">
<h3 class="menu-toggle assistive-text"><?php esc_html_e( 'Menu', 'quark' ); ?></h3>
<div class="assistive-text skip-link"><a href="#content" title="<?php esc_attr_e( 'Skip to content', 'quark' ); ?>"><?php esc_html_e( 'Skip to content', 'quark' ); ?></a></div>
<?php wp_nav_menu( array( 'theme_location' => 'primary', 'menu_class' => 'nav-menu' ) ); ?>
</nav> <!-- /.site-navigation.main-navigation -->
You'll need to copy this into the div's below this code, where it displays the banner. You'll also need to put some conditional checks around it so that it displays on all pages except for the homepage. As well as putting a conditional check around the original code so that it only displays on the homepage.