В нижнюю часть сайта в самый низ вставляем:

Код

<script type="text/javascript">  
$('body').append('<div class="upbtn"></div>');  
$(window).scroll(function() {  
  if ($(this).scrollTop() > 100) {  
  $('.upbtn').css({  
  bottom: '80px'  
  });  
  } else {  
  $('.upbtn').css({  
  bottom: '-80px'  
  });  
  }  
});  
$('.upbtn').on('click',function() {  
  $('html, body').animate({  
  scrollTop: 0  
  }, 500);  
  return false;  
});  
</script>

B CSS:

Код

/*Knopka Verx*/  
.upbtn {  
  opacity: 0.7;  
  z-index: 9999;  
  width: 38px;  
  height: 38px;  
  position: fixed;  
  bottom: -80px;  
  right: 20px;  
  border-radius: 50px;  
  cursor: pointer;  
  border: 2px solid #d2e3f1;  
  transition: all .7s ease-in-out;  
  background-position: center center;  
  background-repeat: no-repeat;  
  background-color: #17527b;  
  background-size: 50% auto;  
  background-image: url(data:image/svg+xml;utf8;base64,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)
}  
.upbtn:hover {  
  opacity: 1;  
  background-color: #337AB7;  
}