Just brew a batch of gray matter. Drill a hole in the skull. Put in the new stuff. Wire it up like the original. Despite its whiff of mad scientist run amok, this scenario is surprisingly close to reality.
Researchers can already do amazing things with mouse brains.
And as they so fondly and frequently point out, mice really are an awful lot like us. Some human experiments already hint at what's possible. Since the 1980s, doctors have cautiously tested transferring brain cells from aborted fetuses to victims of Parkinson's disease. For some, it seems to work remarkably well, restoring lost control of movement. But to those on the cusp of this new technology, Parkinson's is almost too easy.