Customer's username is the email address they provided when registering for a customer account with the store. You can see the customer username as follows:
Go to Customers.
Open customer's details.
You will see the username on the Login section of the page.
Changing the username
The username cannot be edited, but if the customer wants to change it, you can do so as follows.
Changing the password
Sometimes customers may ask you to change their password. For security reasons, you cannot change customers' passwords on their behalf, but you can do the following instead.
Ask the customer to change their password using the Forgot Password feature on the online store's login page.
Send password change instructions to the customer by email. You can do this in the Login section of the customer's account.