If my rework included something incorrect, or incomplete, just click on the Edit link at the bottom of the post, make any necessary changes, and hit Save.
If you want to add something, you can either do that to the original post, or just include it in a new post to the thread.
As the originator of a Classifieds thread, you have more ability to do edits than with normal threads. If, as time progresses, you want to modify the terms of an offer, we much prefer that you leave the original terms in place - possibly adding a strike-out through the text, and then add the new terms with wording such as "New price: $****".