Just to add to my post....
Simply raising or lowering the pitch will probably not sound natural, especially if you're changing it by that much (an octave).
So, while the method I mentioned above will work, it won't sound good.
What I would recommend would be to use ReaPitch. Find the Algorithm that sounds best (Elastique Pro, Soloist, etc...). Also, one thing that helps is to use the "Formants" by setting them to the exact opposite amount of the pitch shift.
What I mean is, if you're raising the pitch by 12 semi-tones, then lower the Formant by 12 semi-tones. Vice versa if you're lowering the pitch by 12 semi-tones, raise the Formant by 12 semi-tones. This might not apply to all the algorithms, but when it works, you'll hear how natural it sounds.
It's not as complicated as I just made it sound. You'll see what I mean once you open ReaPitch.