I had to do this. I had been writing a series for five years when I suddenly felt disconnected from it. How could this happen, I asked myself. How could I possibly lose interest in writing a series that I’ve spent a good part of my life writing?
Sitting back and looking at the whole piece before you dive into rewriting it is the best thing to do. Pull up that file, open that notebook and begin to read it with fresh eyes. As you are reading it you can figure out what you have to do (what needs changing, what characters need to be improved, what direction the story needs to take, etc.), and make a list to keep track of all of your ideas. When you do this, it gives you a clear path to take. Then start recreating a different plot with your new ideas. A detailed plot will reveal whether you have a complete, well thought out book or not.
The key is to have an idea in mind of what you want the book to become before you do reconstructive surgery on it. Otherwise, you’ll get overwhelmed and be left with a shredded piece of work that you may never want to fix.
Taking it one step at a time is always best.