Wordpress add a class to menu link

I need your help. I need to add a specific class on the link that wordpress generates in the menu. What should I edit? The HTML output in my case is:

<nav class="nav" role="navigation">
<ul>
<li id="menu-item-8" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-8"><a href="example.com">Sample Page</a></li>
<li id="menu-item-9" class="menu-item menu-item-type-custom menu-item-object-custom menu-item-9"><a href="http://example.com">Prova sample</a></li>
</ul></nav

the code in the header file that generate the menu is:

    <!-- nav -->
    <nav class="nav" role="navigation">
    <?php html5blank_nav(); ?>
    </nav>
   <!-- /nav -->