how can i set my site title (site branding) to an H1

Ah I see the problem.

Try adding this function:

add_filter( 'generate_site_title_output','tu_blog_h1_title' );
function tu_blog_h1_title() {
	printf(
		'<%1$s class="main-title" itemprop="headline">
			<a href="%2$s" rel="home">
				%3$s
			</a>
		</%1$s>',
		( is_home() ) ? 'h1' : 'p',
		esc_url( apply_filters( 'generate_site_title_href', home_url( '/' ) ) ),
		get_bloginfo( 'name' )
	);
}

didnt work out my site go down !i had to go to cpanel to canael the code .

it is very strange isnt it ?

bummer

Where did you add it? Tested and working fine on my test site.

i have the child theme :
so i inert the code first on the style.css and nothing happened .
then i insert it in the function.php and my site go down .

Try using a plugin like this: https://en-ca.wordpress.org/plugins/code-snippets/