WordPress Menu merchandise > Sub menu Class replace

I am a beginner to WordPress core and development course of. As I’ve a little bit information of WordPress capabilities. However I would like the assistance from you guys.

Really, i am creating my first theme and that i have already got the CSS for my menu. The menu has been registered in capabilities.php.

I’ve used the code posted beneath so as to add my class “mainnav-section”.

<?php 
              $args = [
                'theme_location' => 'header-menu',
                'container'      => 'ul',
                'container_class'=> '',
                'menu_class'     => 'mainnav-section'
              ];
              wp_nav_menu($args);
          ?>

And it executed this.

<ul id="menu-header" class="mainnav-section">
    <li id="menu-item-511" class="menu-item menu-item-type-custom menu-item-object-custom menu-item-has-children menu-item-511">
        <a href="http://#">Newest</a>
        <ul class="sub-menu">
            <li id="menu-item-512" class="menu-item menu-item-type-custom menu-item-object-custom menu-item-512">
                <a href="http://#">HardFork</a>
            </li>
        </ul>
    </li>
    <li id="menu-item-513" class="menu-item menu-item-type-custom menu-item-object-custom menu-item-513">
        <a href="http://#">Improvements</a>
    </li>
</ul>

All I need to to do is so as to add my CSS class to main-menu objects, sub-menu and checklist objects. As a result of I’ve the CSS and JS accomplished to get the desired menu.