Hi
Go to /css/custom.css file and add below code
.back-to-top {background:#b4afaf;bottom: 45px;height: 40px;margin: 0;position: fixed;right: 16px;width: 40px;z-index: 1000;
.transition(opacity 350ms);border-radius:25px;display: none;}
.back-to-top.reveal {opacity(30)cursor: pointer;display: block;}
.back-to-top:hover { background-color: #908e8e; cursor: pointer;}
.back-to-top.active { display: block; }
.fa-angle-double-up:before { color: #fff; font-size: 30px; line-height: 1em; left: 35%; margin-top: -17px; margin-left: -3px; position: absolute; top: 50%; bottom:30px; }
Now open index.php file in template folder Add this code after line 216
<div id="back-to-top" class="back-to-top">
<i class="fa fa-angle-double-up"></i>
</div>
<script type="text/javascript">
//<![CDATA[
(function($){
$(document).ready(function(){
$('#back-to-top').click(function(){
if($(this).hasClass('reveal')){
$('html, body').stop(true).animate({
scrollTop: 0
});
}
});
$(window).scroll(function(){
$('#back-to-top').toggleClass('reveal', $(window).scrollTop() > (Math.min($('#mainbody').length ? $('#mainbody').offset().top : 0, 1500)));
});
});
})(jQuery);
//]]>
</ script>
Here >>> http://prntscr.com/dt808a
(remove the space between / and script in last line of code </ script )
Hope it helps.