Hey, just like the Duck said, you can just click the "pencil" (see pic) next to the chapter and replace pages, etc.
Then you can click "Edit Pages" at the bottom and then you can just delete the page that you want to change and then upload the new one and then just put it where it needs to go in the order it's supposed to go in.
That way some mod won't have to delete the 1st chapter you "replaced".
EDIT: Actually, there's a "Delete Chapter" in the Edit Pages screen. You can just click that. (Gonna leave the rest up in case somebody doesn't know how edit chapters, etc). However, it's preferable to just edit the chapter and stick the new page in so that the comments won't go away when you delete the chapter.