Hello Victor,
You have to do it through the header.php file. Where it says:
<h1 class="blog-title">
<a href="<?php echo esc_url( home_url() ); ?>" title="<?php echo esc_attr( get_bloginfo( 'title' ) ); ?> — <?php echo esc_attr( get_bloginfo( 'description' ) ); ?>" rel="home"><?php echo esc_attr( get_bloginfo( 'title' ) ); ?></a>
</h1>
You can add another line of code, directly underneath this calling the subtitle…
<h1 class="blog-title">
<?php echo esc_attr( get_bloginfo( 'description' ) ); ?>
</h1>
In this case, I have left it in the same styling as the Blog Title, but you can change it to another size (h2, h3, p etc.) and edit the css file to suit your taste!
