@Diane_ODwyerFixing old stories is easy, you just 'repost' them. Even if you cut or add chapters, the new story structure simply replaces the older story, just as the new chapters replace the old ones.
However, if you simply replace a chapter, you won't get much credit for it, and few will notice it. A better approach is to 'retire' the story, let it sit while you rework the old one, and then 'repost' it, especially if you retitle it (while simultaneously marking it "Previously posted as ..."). That way everyone knows that you have revised the story, and they'll be curious what's changed in it. And if there's enough time between the two versions, they won't remember precisely how the first read (i.e. fewer direct compassions of the two).