"Related Posts" - change title to custom title - Add this in Astra child theme's functions.php file

functions.php

add_filter( 'astra_related_posts_title', 'astra_change_related_posts_title' );

function astra_change_related_posts_title( $title ) {
	$custom_title = 'Your Custom Title';
	return str_replace( 'Related Posts', $custom_title, $title );
}