So here I am looking at 87,000 words -- a year's worth of writing and I want to tease a novel out of it. It's a pretty straight forward linear novel. I've been planning to read the whole thing through first, then manipulate it into shape, but I found I wanted to rewrite chapter one first to set the scene and get the thing going. So I did that, then chapter two. But I find that I'm not really changing much, only correcting spelling, eliminating repetition, etc. The biggest thing is that I'm cutting a lot of rambling background and description out, trying to get to the bones of the story so that it moves fluidly. I suppose there are as many ways to edit a book as there are writers of them. It's like curry recipes in India. How many are there? What do you do? Rewriting I mean, not curry, but if you have a curry recipe feel free to pass that on too.
Look for plot holes and inconsistencies. There are generally a lot. Some are easy to fix (were her eyes blue or green) and some are less to (so Frank couldn't have been there to witness the murder because the same day he was at his daughters recital on the island, and the ferries don't run at night, but I need him to be there for the emotional arc, but if he's a witness then Sally won't be able to get this clue off him, and yes it has to be him, etc.)
Don't look at your first draft when making the second draft unless readers pointed out a specific section as a poetic part to keep. Try writing everything from memory as if you wrote it for the first time. The style will improve significantly each time and if you forgot something from the first draft then it wasn't memorable enough in the first place, unless it's an introduction of a concept you're using. First draft: Get to know many characters while quickly going forward. This creates the world in a thought experiment. Second draft: Simplify, remove anything that's not good enough, try new things. Get feedback from random people and establish your target audience. Third draft: Polish and flesh out with more details. Get beta readers within the group that are still honest and constructive.
If you're not changing much, maybe it doesn't need to be changed. Just don't take what you have for granted. Ask yourself if your characters are behaving consistent to their world, their roles, and their own natures. Treat even walk-ons as real people, not as shadow puppets. Failing to do so can end you up in a real chasm of a plot hole. Or several.