That did the trick. Thanks :)

replace DESC by ASC

(see https://www.w3schools.com/sql/sql_orderby.asp)

Hi. The thing is it already looks like this:

                $vars = array(
                    'type' => 'forum:topic:reply',
                    'order_by' => 'a.id DESC',
                    'subject_guid' => $subject_guid
            );

but it still show from newest to oldest.

Hi. I can't program. What should I change it into?

OK, change into what?

You may change that in components/Forum/classes/Replies.php
change $vars array with for descending order :

            $vars = array(
                    'type' => 'forum:topic:reply',
                    'order_by' => 'a.id DESC',
                    'subject_guid' => $subject_guid
            );