Try adding this PHP Snippet:
add_filter( 'generate_site_title_output', function(){
return sprintf(
'<%1$s class="main-title"%4$s>
<a href="%2$s" rel="home">
%3$s
</a>
</%1$s>',
( is_front_page() ) ? 'h1' : 'p',
esc_url( apply_filters( 'generate_site_title_href', home_url( '/' ) ) ),
get_bloginfo( 'name' ),
'microdata' === generate_get_schema_type() ? ' itemprop="headline"' : ''
);
} );