...you make a new one.
That's what I'm doing with Mars. I wasn't happy with the story as planned out, so I'm totally re-vamping the thing. That's the great thing about outlining, though--you can go in a completely different direction before committing to putting a lot of words on the page.
Better to have a bad outline than a bad book.
I've also started making notes for my next series. I had thought about doing a fantasy series, but it looks like that won't happen yet. The next one, like the Free Space trilogy, will also be science fiction.
Unlike Free Space, though, it won't be hard SF. And it will have a metaphysical/supernatural component.
Of course, all plans are subject to change at any time. ;)