To add a favicon to your website, please upload an appropriate image in .ico extension to the root folder of your shop. If this does not help, then open your index.html file and insert the below string somewhere at the beginning:
<link rel="shortcut icon" href="http://www.yoursite.com/image.ico" type="image/x-icon" />
where www.yoursite.com is your site URL
and image.ico is your image for the site.