Hello @pixelshrink ,
The free Kadence theme does not have options to change the single post titles from an h1 to an h2. However, you can accomplish the task using a code snippet. You can add PHP code to your site using a plugin like Code Snippets or your functions.php file (in a child theme).
For example, here is code that should work with the Kadence theme.
add_action('wp_footer', function() {
?>
<script>
jQuery(document).ready(function($) {
$('h1.entry-title').replaceWith(function() {
return $("<h2 class=entry-title>").append($(this).contents());
});
});
</script>
<?php
}, 20);