Convert even byte array to odd

[scidam]

This represents a large number which is even. I want to convert this to an odd number as efficiently as possible.

This needs to be clarified.
Let we have a bytestring, e.g. x = b'\x12\xfe'. This bytesting represents an even integer,

>>> int.from_bytes(x, byteorder='big')

Output:
4862

Now, you want to turn this integer out to some odd integer. Ok, the fastest way to do this
is reassigning, e.g. x = 1. It is not you are looking for, isn't it?
So, you need to clarify the problem. You are probably looking for a method which allow you to get
closest odd number to the current number, aren't you?. The following is example that helps you:

>>> x = 839120938198492
839120938198492
>>> x.to_bytes(10, byteorder='big')
b'\x00\x00\x00\x02\xfb-\x11q\xe5\xdc'
>>> z=x.to_bytes(10, byteorder='big')
>>> z
b'\x00\x00\x00\x02\xfb-\x11q\xe5\xdc'
>>> int.from_bytes(z, byteorder='big')
839120938198492
>>> closest_odd = z[:-1] + (z[-1] - 1  if z[-1] % 2==0 and z[-1] - 1 > 0  else z[-1] + 1).to_bytes(1, byteorder='big')
>>> int.from_bytes(closest_odd, byteorder='big')
839120938198491