The main menu can be customized from app/index.html > body.

Each menu item has the markup

<li><a class="active" href="index.html"><span>Home</span></a></li>

<ul class="cg__mainMenu clearfix">
  <li><a class="active" href="index.html"><span>Home</span></a></li>
  <li><a href="studio.html"><span>Studio</span></a></li>
  <li><a href="our-process.html"><span>Our process</span></a></li>
  <li><a href="work.html"><span>Work</span></a></li>
  <li><a href="news.html"><span>News</span></a></li>
  <li><a href="contact.html"><span>Contact</span></a></li>
 </ul><!--/.cg__mainMenu-->

The css for the menu can be changed in app/css/scss/_site-header.scss