It is easy to achieve since the theme is based on templates and you can override the templates by copying them from the parent theme to the child theme. Pls override the template – wp-content/themes/appdev/templates/header/site-branding.php and change this line –
$heading_tag = (is_home() || is_front_page()) ? 'h1' : 'div';
to
$heading_tag = 'div';
To change to H1 tag for the post title on the blog page, you can copy this template –
wp-content/themes/appdev/templates/archive/content-entry-title.php
and then change the H2 tag used there to H1. Hope this is clear.