This article will teach how to change an existing user's Username in two ways.
Instructions
Method #1
Go to Settings → CRM Settings → User Management
Click on the Info Icon Next to the user Record and Select “Change Username.”
Add the New Username and click Save.
Method #2
Go to Settings → CRM Settings → User Management.
Select the user record for which you would like to change the username and Click the “More” Icon on the top right of the user record selected.
Select “Change Username,” Add the New Username, and click Save.
"When changing an existing user to a new one, it's important to update all critical information such as the username, password, email, and other relevant user details.”