If you want to change the Post title tag then first you need to create a Custom Plugin and add below the filter.
Example code:
add_filter('ampforwp_modify_title_tag','prefix_amp_title');
function prefix_amp_title($title){
$title = '<h2 class="amp-post-title">' . wp_kses_data( get_the_title(ampforwp_get_the_ID()) ) . '</h2>';
return $title;
}The Output of Post title tag in AMP

That’s it for this tutorial!
We hope it helped you. If you have any feedback or questions, then please share them in the comments section below.
Does this hook still exist? cause it dosent work form me. thanks
Hi Federico Cadierno,
Will you please contact our technical team from here- https://ampforwp.com/support/ they will help you as soon as possible.
Hi,
This is very strange, I installed AMP on a staging site and a live site but for some reason the staging site title tag is correctly reading the tag but my live site for some reason picks . Any idea how this is possible? Is there a file I can access to check the default AMP tag?
Hi Mubin,
Will you please contact our technical team from here- https://ampforwp.com/support/ they will help you as soon as possible.