<nav>
<ul>
<li><a href="/" class="active">Home</a></li>
<li><a href="/collections/all">Books</a></li>
<li><a href="/blogs/five-simple-steps-blog">Blog</a></li>
<li><a href="/pages/about-us">About Us</a></li>
<li><a href="/pages/support">Support</a></li>
</ul>
<select>
<option value="" selected="selected">Select</option>
<option value="/">Home</option>
<option value="/collections/all">Books</option>
<option value="/blogs/five-simple-steps-blog">Blog</option>
<option value="/pages/about-us">About Us</option>
<option value="/pages/support">Support</option>
</select>
</nav>
And here is the CSS. Nothing complicated here: We hide the <select> by default, and only show it if the document width is smaller than 960px.
nav select {
display: none;
}
@media (max-width: 960px) {
nav ul { display: none; }
nav select { display: inline-block; }
}