How to edit an .epub file?

So I have an.epub file, and it doesn't have any table of contents included in it, and on my iPhone, when I read it, it doesn't tell me how many pages there are left until the next chapter, just how many until the end of the book. I was wondering if you could somehow edit it to include a table of contents, and links to the various corresponding pages. It's just...... rather annoying.