As most authors my know, a single book may take several dozen readings before it is ever finished. This is an arduous task that isn't to be taken lightly, and can sometimes be the difference between quitting and publishing.
Now, some books, I will admit, you just need to be quit. Start something new. But for those who do not, editing can be a rewarding task.
Editing is, probably, one of my favorite parts about writing--well, that's not true, I love all parts of it, from the idea stage up to and including publication. But editing has a special place in my heart. I get to shape a story from being okay to good, and from good to . . . a little higher.
It is also a HUGE hurdle. More time is probably spent editing your book than any of the other hurdles combined, with the exception of waiting on publication--traditional, that is. But that hurdle will have to wait.
How do you edit?
To each their own. I know that's a crappy answer, but each writer takes to editing a bit differently. I could tell you how I do it, and how it has evolved. For that, just click on the "editing" tag along the bottom of this post for various tips I've used.
I do have one suggestion: read it aloud. If you stumble when you're reading it, you might want to rewrite the passage. Constantly be asking questions about the story, on how to improve it.