[Theme: Custom] Menu: adding classes to inner tag

Hi,

Quick question:
Does anyone know how to add classes to WordPress menu items?
I know you can add classes to

• elements, but is there a way to add classes to the inner tag?

Is this possible?

<nav>

  <ul id="menu level1">

      <li class="current-menu-item"><a href="#" class="fa fa-home">item</a></li>

      <li class="has_submenu"><a href="#" class="fa fa-desktop">item</a>
          <ul class="submenu level2">
            <li><a href="#">submenu_item</a></li>
            <li><a href="#">submenu_item</a></li>
            <li><a href="#">submenu_item</a></li>
          </ul>
      </li>

      <li class="has_submenu"><a href="#" class="fa fa-pencil">item</a>
          <ul class="submenu level2">
            <li><a href="#">submenu_item</a></li>
            <li><a href="#">submenu_item</a></li>
            <li><a href="#">submenu_item</a></li>
          </ul>
      </li>

      <li><a href="#" class="fa fa-eye">item</a></li>

  </ul>				

</nav>

If this is possible, does anyone know how to do this?

Simplefied HTML (removed title attributes etc.): CODEPEN (http://codepen.io/anon/pen/aCjvI?editors=100)

Thanks in advance!