tom_laan84
What Pavit suggested above is the article title, if you look in code of the same file, you can see that we use H1 tag for the page heading:
<?php if ($this->params->get('show_page_heading')) : ?>
<div class="page-header">
<h1> <?php echo $this->escape($this->params->get('page_heading')); ?> </h1>
</div>
<?php endif; ?>
so we use H2 for the article title, of course, you can change as you wish.
And we don't recommend to edit in core file, to override this file, you can copy and paste in this override path inside local folder of template you're using:
/templates/t4_blank/local/html/com_content/article/default.php
If this path doesn't exist, simply create it.