You are barking up the wrong tree. By ipub, I assume that you mean ePub. The electronic book format ePub is a superset of HTML. Thus, the content of an ebook is much more like that on a web page than a PDF document. Assuming that your ipub file is not copy-protected, there are a number of ePub editors available for the Mac. You may find them here. Take a close look at Sigil 0.8.3.
If you want an iPad-based ePub editor, then I don't see any in the App Store.