>>> import ctypes
>>> b = b'ABC'
>>> b
b'ABC'
>>> ctypes.cast(b, ctypes.POINTER(ctypes.c_char))[1]
b'B'
>>> ctypes.cast(b, ctypes.POINTER(ctypes.c_char))[1] = b'X'
>>> b
b'AXC'

byte列ならうまくいくが
stringでやるとコンソールが飛んだわω