Readers are fickle. When your latest book is fresh and new, they stop by in droves and you get the highest sales results. As the weeks and months go by your sales will diminish until it reaches its natural stopping point. If you've been writing for a while you've probably got a list of older books in this dormant condition. While the best way to increase sales may be to write another book, you can still add to your income by rejuvenating one of your older titles.
In the movie Groundhog Day, Bill Murray spent day after day improving his life, using clues he'd found in days he'd just experienced. The same day, only slightly improved, always gave him better results. Take one of your older titles and do the same thing, and you can freshen it up enough to look good to newer readers who never found it the first time.
Most authors that self publish can look back and find at least one cover they want to change. The old one was based on short-lived trends, the style looks dated, or the tropes in their genre have changed. Creating a new cover, or having one made, is probably the best way to revive an older book in your catalog. Research the top 100 sellers in your niche and find out the popular themes in covers there, then create a completely new look to your old book.
That little elevator speech on the back of your book or in the middle of your online sales page is a crucial piece of marketing, but many writers treat it as a simple throwaway to be written in five minutes. Study the blurbs of bestselling books to find out how they work and why, then redo yours to follow those rules.
Your grammar can be flawless but, unless your book was written in the past year, you could have details in the book that feel out of style with today's readers. Does your detective constantly check his Blackberry? Is your book set in modern times, yet your main characters are addicted to watching Friends? Most readers won't care, but for the ones who do, it will be supremely annoying, probably enough so that they leave a bad review. Edit your book with an eye to modern details, slang, trends, and current events.
Just like in the movie, eventually you'll get it all right and have exactly the book you were meant to have. Doesn't that deserve a splashy new marketing campaign? Hold a release party on Facebook. Give away copies on Rafflecopter. Make up bookmarks and other swag and give it away to random fans and street team members. If you've got at least 10 reviews, try to get the book on Bookbub or one of the other big review sites. This is your second chance at marketing, and the seasoning your book's had can make it an even tastier read than it was the first time around.
Pulling a book and republishing a newer edition isn't designed to fool readers into buying an old book all over again. In fact, you should add a prominent disclaimer, stating that the book was previously published under another name, and that this new issue includes content not previously published. When Stephen King did this with The Stand, he sold even more copies than with his original work. Hopefully you'll have a fraction of the success he did.
Keep the Faith and May the Force be With You!