what would it be, and why. Only one thing. For example, if you changed your age, that's it. You're a younger you, with no other changes. If you change gender, that's it - you'd be a female version of yourself at your current age.
As for me, I'd make myself hyper-intelligent. Really, really super-intelligent, enough to find cures for cancer (and get rich), and create either nanobot technology or an MAU. Then I could change more about myself. And I could use the technology to help my family improve, live longer, etc. Nanobots could help people change things they needed to - like in many stories, live longer, prevent diseases, etc. (Yes, I know it's like wishing for more wishes, but what the heck?)
Not that I've ever given this much thought :)