The first time she submitted Flowers in the Attic, V. C. Andrews was rejected. It was too long, the publisher advised, and boring. So Andrews went back to the book, eliminated about 100 pages and added the stuff that would "make [her] mother blush." It was a huge hit right away, leading to a movie, three sequels, one prequel...and a career that outlived the author herself.
Authors don't always get it right the first time, especially self-published authors who have to learn how to do every bit copyediting, formatting and marketing on their own. There are lots of reasons why you might have to issue a re-release or a new edition of a book you've already published in the past. But if you do it, make sure you do it the right way.
Re-releases are common in the book world, and there's no reason self-published authors can't join in. Books that have been re-released usually are not changed in any way; all the text of the book is the same. In some cases, the text may be newly copyedited for correctness. But even in this case, the story hasn't changed; nothing has been added, or taken away.
That's why you've got to add one crucial element to make your re-release more successful: you need a new cover. You have to bring something new to the table, and you have to distinguish this new re-release from the old version of your book. An introductory price definitely wouldn't be out of line, either. Promote the new cover, the temporary price and the re-release itself.
If any significant changes have been made to the book, including very thorough editing, you have to release a new edition (in most cases, the Second Edition). It's okay to stick with the same cover if you like, because when you put out a new edition you won't be creating a brand-new page for the book. You will be wholly replacing the first edition of the book, so the cover can carry over if you like.
However, you'll have to promote the new edition really hard. With self-published authors, new editions are most commonly released because major editing errors or formatting problems have now been corrected. This is a great thing to do; if there are problems with your book, fix them. If you've decided to address a bunch of problems at once and release a whole new edition, you've got to make a whole thing out of it.
Why? Because, chances are pretty good that you got several negative reviews regarding the editing and formatting (why else would you go to such trouble to fix it all?), and now you've got to win readers back. You'll have to respond to the negative reviews, and announce on your book's information that this second edition has been fully edited and re-formatted, or whatever.
To make the new edition even more palatable, and perhaps get some second-time buyers as well, you might want to include something else: an author's note. It's promotable, and it's a good way to address the readers directly and tell them why you've released this new edition of the book. Since significant changes have been made, it's a good idea to reference these changes and re-invite your fans to read the story.
Of course, there's another really important reason you might issue a re-release of your books: you wrote a series. It's common practice in the book biz to release boxed set editions of serial books; I personally own several of these sets. Self-published authors are in the book biz, so why not release a boxed set of your series?
If you do, it's a good idea to give the fans a reason to re-buy all the books they already read. You're going to need to add extras. Did yo draw up any sketches while you were working on the books? Maybe create some maps, calendars, notes? What research materials did you pull? Take a look back through all your notes, and you might find some interesting tidbits that you can share with your readers. Add a few extras to your re-releases and new editions, no matter why you're releasing them, and you'll give readers that much more incentive to check out your new-old book. You're trying to get a second chance, so make it as easy as possible for them to give you one.