How do you replace a dictionary key with a new input?

[thewetmosquito]

Good afternoon Python community,

I am practicing my Python skills by working on a log-in/registration script that asks if you are currently registered. If Y for 'Yes', then it runs the user through a log in validator. If 'No' then it runs the user through a sign-up form that validates each input field, and stores it into a dictionary userBase. At the end of the sign-up form, the program lists out all of the fields that were filled in to verify if the information is correct. If Y for "Yes" the program exits. If "N" for no, the program asks the user to choose which field they want to update, and re-prompts the field for them to update the selected information. This will loop until the user chooses Y for "Yes", and the program will exit.

My problem occurs when the user is asked to update a field. In a perfect world, the updated information will replace the appropriate dictionary key/value. However, I have not wrapped my finger around how exactly can I go about doing this. The original program is about 140 lines long, so I isolated the problem to 14 lines in order for you to see what I'm talking about:

import pprint

username = input('enter username: ')
password = input('Enter a password: ')
userBase = dict(
    user = {
        username:{'password': password}
        })


updateUsername = input('Enter in your new username: ')
userBase['user'][username] == updateUsername

pprint.pprint(userBase)

The output that I'm getting is:

Output:
enter username: John
Enter in your new username: Jeff
{'user': {'John': {'password':'12345abcde'}}}

The desired output is:

Output:
enter username: John
Enter in your new username: Jeff
{'user': {'Jeff': {'password':'12345abcde'}}}

I have searched around the internet for strategies on how to replace a dictionary key/value without entering an immutable string value, but nothing has worked for me so far. Is this something that I have to hack? Am I overlooking something that's very simple?