The thread you responded to hasn't been updated since 2006. Our system has had many updates since then.
Here are the instructions on how to update your subject line as of today.
If you are not in the email anymore:
Go to the Email tab > under My Emails. Click on the name of your email that you want to edit the subject line for. You will then see this:
You can edit the subject line there.
If you are in the email still and have saved all your changes, click on the Continue button on the top right. That will bring you to the same page as above.
I hope that helped!