How to remove sticky header?

Take a look for this on line 1174 in your style.css –

.site-header {
	background-color: #000;
	left: 0;
	position: fixed;
	top: 0;
	width: 100%;
	z-index: 999;
}

You can remove/adjust the transparent background on line 1197 –

.featured-section .site-header {
	background-color: transparent;
	border:  none;
}

You can adjust the padding-top on line 1193 if you want to remove the space at the top –

.front-page .site-header {
	border-bottom: 1px solid #ddd;
	padding-top: 30px;
}