// Filter for changing related posts title heading tag.
add_filter( 'astra_related_posts_title', 'update_related_posts_title_markup', 10, 1 );
function update_related_posts_title_markup( $markup ) {
    $markup = '<div class="ast-related-posts-title-section">';
     $markup .= '<h4 class="ast-related-posts-title">'; // Replace H4 with the tag you need.
 $markup .= 'Related Posts';
 $markup .= '</h4>'; // Replace H4 with the div you need.
    $markup .= '</div>';
    return $markup;
}

But I am unable to change the particular related post heading tag