functions.php

<?php

/**
 * Primary Nav using Dropdown Walker
 *
 * @param array $args
 * @return array
 */
function be_primary_nav_dropdown( $args ) {
	if( 'primary' !== $args['theme_location'] || !class_exists( 'Walker_Nav_Menu_Dropdown' ) )
		return $args;

	$args['depth']= 1;
	$args['walker'] = new Walker_Nav_Menu_Dropdown();
	$args['items_wrap'] = '<div class="mobile-menu"><form><select onchange="if (this.value) window.location.href=this.value"><option value="">Go To&hellip;</option>%3$s</select></form></div>';
	
	return $args;
}
add_filter( 'wp_nav_menu_args', 'be_primary_nav_dropdown' );