You can easily use .replace() as also previously described. But it is also important to keep in mind that strings are immutable. Hence if you do not assign the change you are making to a variable, then you will not see any change.
Let me explain by;
>>stuff = "bin and small"
>>stuff.replace('and', ',')
>>print(stuff)
"big and small" #no change
To observe the change you want to apply, you can assign same or another variable;
>>stuff = "big and small"
>>stuff = stuff.replace("and", ",")
>>print(stuff)
'big, small'