You can try adding the following codes:
function mycustom_comment_form_title_reply( $defaults ) {
$defaults['title_reply'] = __( 'Leave a Comment' );
return $defaults;
}
add_filter( 'comment_form_defaults', 'mycustom_comment_form_title_reply' );
You can add these codes by using plugins such as the WpCode instead of adding them to functions.php to prevent the file bring overridden when upgrading.