how to change my back to top button with an arrow

hi. I created a "back to top" button for my website. it's just a circle actually. I want to add an arrow into the circle to make it more clear.  

here is my codes:

 

-TO THE HEADER-

<a href="#" class="btt"> <span class="btt-icon"></span> </a>

- TO THE FOOTER -

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script>
  var scrolled = 400; $(window).scroll(function() {
    if ( $(window).scrollTop() > scrolled ) {
      $('a.btt').fadeIn('slow');
    } else {
      $('a.btt').fadeOut('slow');
    }
  });
</script>
<script>
  $('a.btt').click(function() {
    $('html, body').animate({ scrollTop: 0 }, 700);
    return false;
  });
</script>

- CSS -

a.btt {
  display: block;
  position: fixed;
  z-index: 999;
  width: 45px;
  height: 45px;
  right: 30px;
  bottom: 30px;
  background: white;
  -webkit-border-radius: 30px;
  -moz-border-radius: 30px;
  border-radius: 30px;
  box-shadow: 3px 3px 10px rgba(23, 25, 25, 0.2);
  transition: transform 0.3s ease-in-out;
}

My testing

5 minutes ago, Beyondspace said:

You can add the following code to get the arrow

a.btt {
  background-image:  url('https://api.iconify.design/material-symbols/arrow-upward.svg?color=%23626262');
  background-repeat: no-repeat;
  background-position: center center ;
  background-size:contain;
}

 

thanks!