you could simply use CSS via a ‘custom CSS’ plugin to hide the top menu;
example:
#container3 .navtop { display: none; }
or more elegantly create a child theme and edit header.php in the child theme, and remove this section:
<div id="container3">
<div class="navtop">
<?php wp_nav_menu( array( 'theme_location' => 'header-menu', 'fallback_cb' => 'wp_page_menu', 'container' => 'div', 'container_class' => 'navmenu', 'menu_class' => 'navmenu', 'show_home' => 'Home' ) ); ?>
</div>
</div>
for details, please consider to ask your question in your theme’s forum at https://wordpress.org/support/theme/newbasic#postform