H1 H2 h3 size

Hi,
For your first question, changing the iconbox title to H1, we can help you with this with a custom function, but first, are you sure you want to do this? Because for good SEO each page should only have one H1 & H2, you can have many H3. You can change the font size of your headings to any size with no issues, but you should still only have one H1 on each page.
Now for the second part of your first question, it looks like you are using Enfold v4.5.3 without a child theme, so any customizing functions we offer will be lost when you update to our latest version 4.6.2 I would recommend installing the child theme, so your changes will not be lost with future theme updates Read about it & Get it here
For your second question about the font size of your iconbox title, it is not set by the heading size, H3, it is set by the “.iconbox_content_title” class, to change this please try this code in the General Styling > Quick CSS field or in the WordPress > Customize > Additional CSS field:

#top #wrap_all .iconbox .iconbox_content .iconbox_content_title {
	font-size: 20px !important;
}

please adjust the font size to suit.

Best regards,
Mike

Hi,
Very good, glad you are using a child theme. Try adding this code to the end of your child theme functions.php file in Appearance > Editor:

function replace_tags_with_tags(){
  ?>
  <script>
    (function($) {
    	$(document).ready(function(){
        function replaceElementTag(targetSelector, newTagString) {
          $(targetSelector).each(function(){
            var newElem = $(newTagString, {html: $(this).html()});
            $.each(this.attributes, function() {
              newElem.attr(this.name, this.value);
            });
            $(this).replaceWith(newElem);
          });
        }
        replaceElementTag('h3.iconbox_content_title', '<h2></h2>');
    	});
    }(jQuery));
    </script>
  <?php
  }
add_action('wp_footer', 'replace_tags_with_tags');

It will change all h3 iconbox titles to h2
You don’t need to update if all is working fine for you right now.
If you have trouble with this please ensure that your jQuery is loading in the header, that is uncheck Enfold Theme Options > Performance > Load jQuery in your footer

Best regards,
Mike