By default, WP requires each new user to enter a "username" in order to create a new account.
How to change this "username" field to email? So, new users only need to enter their email during the sign up process; after they verify their email address, new users will use email address to login to WP.
In other words, eleminate the "username" and replace it with email address for 1) new users sign up; 2) users login.
Thank you for reading my question and look forward to any tips or solutions