dropmenu in underscores

How Can I do dropmenu in underscores…

if I have static html/css/js – I can simply add new classes in

tag:

—–static html/css/js————-

<ul class="dropmenu">
	<li><a href="#">link</a></li>
	<li><a href="#">link</a></li>
	<li><a href="#">link</a>
			<ul class="dropmenu"><li><a href="#">drop items1</a></li>
						<li><a href="#">drop items1</a></li></ul>
	</li>
</ul>

——————

but in underscores, I don’t see where and how…

———dynamic index.php—————-

<!--nav-->
		
<nav class="nav"><button class="menu-toggle" aria-expanded="false"><?php esc_html_e( 'Primary Menu', 'underscores' ); ?></button>
	<?php wp_nav_menu( array( 'theme_location' => 'primary', 'menu_id' => 'primary-menu' ) ); ?>
			
		</nav><!-- #site-navigation -->

——————————-

maybe In functions.php file?

best regards