How can I correct an error in an old post?

[barakad at gmail.com]

Hi.
I was searching for some information regarding a problem and found an
interesting post that includes an answer to the problem; thought the
post is very helpful it is based on a wrong assumption and thus the
solution it suggests is incorrect. It took me some time to understand
that the suggested solution is wrong and to find the correct solution
and I wish to add my findings to the post, to prevent others from
taking the wrong path.
When I tried to replay to the post I received a reject message stating
that it is impossible to replay to the topic since it is old and was
closed by a manager.
The question is how can I add this correction?
Thanks.