Python strings are immutable, you change them by making a copy.
The easiest way to do what you want is probably.
text = "Z" + text[1:]
The text[1:] return the string in text from position 1 to the end, positions count from 0 so '1' is the second character.
You can use the same string slicing technique for any part of the string
text = text[:1] + "Z" + text[2:]