How to edit characters of a string?

Hey, I was trying to edit a string on Python IDLE but it threw me an error:

>>> text = 'kar'
>>> text[0]='c'
Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
TypeError: 'str' object does not support item assignment

How can I edit my string? Can it be even done using indexing? Please help me out.