You need to alter the template files under the theme folder. In the folder template-parts
there is content-page.php
that renders the page (well, at least the part with the header) and content-single.php
that has the single post’s title part.
Just change the <h2>
and </h2>
tags to be <h1>
and </h1>
respectively on the <?php the_title( '<h2 class="entry-title">', '</h2>' ); ?>
part.