jekyll/lib/site_template/_includes/header.html

<header class="site-header">

  <div class="wrap">

    <a class="site-title" href="/">{{ site.name }}</a>

    <nav class="site-nav">
      <a class="page-link" href="/about">About</a>
      <a class="page-link" href="/projects">Projects</a>
    </nav>

  </div>

</header>