scrollToTop-script.js

jQuery(document).ready(function($){
    var offset = 700;
    var speed = 250;
    var duration = 500;
        $(window).scroll(function(){
            if ($(this).scrollTop() < offset) {
                      $('.topbutton') .fadeOut(duration);
            } else {
                      $('.topbutton') .fadeIn(duration);
            }
        });
     $('.topbutton').on('click', function(){
            $('html, body').animate({scrollTop:0}, speed);
            return false;
            });
});


//Text to be placed for icon makrup (FontAwesome Required)
<a href="#" class="topbutton"><i class="fa fa-chevron-circle-up"></i></a>

This script (along with some other code in functions.php) will add Scroll to top feature in a WordPress site