You cannot!

Strings in Python are immutable (something that cannot be changed)

Why are Python strings immutable?

Read here

What can you do?

You can create a new modified string.

Examples

Convert all characters to upper case

original = "My String" 
new_string = original.upper() 
print(new_string)  # "MY STRING"

Change one character

Since you can't really change the string, the solution is to convert it a mutable type like list and modify that.

original = "My String" 
original_list = list(original)
original_list[0] = 'm' 
new_string = "".join(original_list)
print(new_string)  # 'my String' 

You could also try slicing to speed up the process