Use the Code Snippets plugin to add:

function update_site_settings() {
    $screen = get_current_screen();

    if (strpos($screen->id, 'your-options-page-slug-here') == true) {
        $blogname = sanitize_text_field(get_field('your_site_title_field_name_here', 'options'));
        update_option('blogname', $blogname);
        
        $blogdescription = sanitize_text_field(get_field('your_site_tagline_field_name_here', 'options'));
        update_option('blogdescription', $blogdescription);
    }
}

add_action('acf/save_post', 'update_site_settings', 20);

What’s not covered is keeping the General Settings WordPress option page fields in sync with the ACF fields. If you’re creating a simplified UI for a client or end-user, you might want to consider hiding the Settings option page in the menu.