Menu Items

Hi – My website is https://www.pearsonkoutcherlaw.com and my current menu is as follows:

<div>
    <div id="nav">
        <div class="nav-borders">
            <?php //wp_nav_menu( array( 'theme_location' => 'primary', 'menu_class' => 'nav-menu' ) ); ?>

            <ul id="menu-main-navigation" class="menu">
                <li id="menu-item-738" class="menu-item menu-item-type-post_type menu-item-object-page current-menu-item page_item page-item-359 current_page_item menu-item-738">
                    <a href="<?php echo site_url(); ?>">Home</a>
                </li>
		<li id="menu-item-416" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-416">
                    <a href="<?php echo site_url(); ?>/firm-overview/">Firm Overview</a>
                </li>
                <li id="menu-item-419" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-419">
                    <a href="<?php echo site_url(); ?>/lehigh-valley-work-comp-attorney/">Lehigh Valley</a>
                </li>

                <li id="menu-item-418" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-418">
                    <a href="<?php echo site_url(); ?>/our-philadelphia-workers-compensation-attorneys/">Our Attorneys</a>
                </li>
                <li id="menu-item-628" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-628">
                    <a href="<?php echo site_url(); ?>/recent-settlements/">Recent Settlements</a>
                </li>

                <li id="menu-item-417" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-417">
                    <a href="<?php echo site_url(); ?>/blog/">Blog</a>
                </li>
                <li id="menu-item-361" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-361">
                    <a href="<?php echo site_url(); ?>/contact-us/">Contact Us</a>

            </ul>
        </div>
    </div>

I do not understand how to add a drop down to this menu and was wondering if someone could help point me in the right direction?

Thanks!