Providing you know the index value of the beginning and end of each word you wish to replace in the character array, and you only wish to replace that particular chunk of data, you could do it like this.
>>> s = "papa is papa is papa"
>>> s = s[:8]+s[8:13].replace("papa", "mama")+s[13:]
>>> print(s)
papa is mama is papa
Alternatively, if you also wish to retain the original data structure, you could store it in a dictionary.
>>> bin = {}
>>> s = "papa is papa is papa"
>>> bin["0"] = s
>>> s = s[:8]+s[8:13].replace("papa", "mama")+s[13:]
>>> print(bin["0"])
papa is papa is papa
>>> print(s)
papa is mama is papa