<a href="registerform.php"> <button type="button" name="registbttn" class="registbttn" onClick="refresh('registerform.php')">REGISTER</button></a>

<script type="text/javascript" src="js/jquery-3.4.1.min.js"></script>
<script type="text/javascript">
    $(document).ready(function(){

        $('#contents').load('home.php');

    });

    function refresh($linked){
            $link = $linked;
            $('#contents').fadeOut('slow').load($link).fadeIn('slow');
    }

</script>