Widget Title – Change from

Try it with this in your (child) theme’s functions.php file

function related_posts_widget_title( $params ) {
	$widget = isset( $params[0]['widget_name'] ) && ( 'Related Posts By Taxonomy' === $params[0]['widget_name'] );

	if ( !$widget ) {
		return $params;
	}

	$params[0]['before_title'] = '<h6 class="widget-title">';
	$params[0]['after_title']  = '</h6>';

	return $params;
}

add_filter( 'dynamic_sidebar_params', 'related_posts_widget_title' );

btw:
consider creating a child theme instead of editing your theme directly – if you upgrade the theme all your modifications will be lost. Or create a plugin with the code above.