Can one rename a database user (schema)

No, this is listed as Enhancement Request 158508

Workaround
Do a user – level export of user A

Create new user / schema by name of B
Import system/manager fromuser=A touser=B

SQL>Drop user A

How to fix: ERROR 1045 (28000): Access denied for user ‘root’@’localhost’ (using password: YES)

Open your terminal and type mysql -u root -p Enter your password. Hopefully your MySQL is logged in now.