How to control time of scroll to top

Question

I'm wondering how to control the time of following scrollTop() function. If i press the button, It goes back to top in a ligtining way. I want to make it slower so it can be presented in a better way. I'm new to javascript and i tried to apply duration which didn't work [I may have applied it in a wrong way]. Is there something to control time of the following code with only additions in the below code.

//Get the button
var mybutton = document.getElementById("myBtn");

// When the user scrolls down 20px from the top of the document, show the button
window.onscroll = function() {
  scrollFunction()
};

function scrollFunction() {
  if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
    mybutton.style.display = "block";
  } else {
    mybutton.style.display = "none";
  }
}

// When the user clicks on the button, scroll to the top of the document
function topFunction() {
  document.body.scrollTop = 0;
  document.documentElement.scrollTop = 0;
}

body {
  font-family: Arial, Helvetica, sans-serif;
  font-size: 20px;
}

#myBtn {
  display: none;
  position: fixed;
  bottom: 20px;
  right: 30px;
  z-index: 99;
  font-size: 18px;
  border: none;
  outline: none;
  background-color: red;
  color: white;
  cursor: pointer;
  padding: 15px;
  border-radius: 4px;
}

#myBtn:hover {
  background-color: #555;
}

html {
  scroll-behavior: smooth
}

<button onclick="topFunction()" id="myBtn" title="Go to top">Top</button>

<div style="background-color:black;color:white;padding:30px">Scroll Down</div>
<div style="background-color:lightgrey;padding:30px 30px 2500px">This example demonstrates how to create a "scroll to top" button that becomes visible
  <strong>when the user starts to scroll the page</strong>.</div>