There's no direct way to rename an user, simply because Oracle does not support it. A supported way is to DROP USER then CREATE USER with a new name.
If you insist to change it, you may consider to update SYS.USER$, a table which persists users' data. But this way is not supported by Oracle.
SQL> show user
USER is "SYS"
SQL> update user$ set name = 'NEW_NAME' where name = 'OLD_NAME';
1 row updated.
In the above, an uppercase name is usually used for your new name.
Please note that, changing a schema name like this way could be risky to your business operations, any objects related to this user should be inspected, modified, rearranged and tested before publicly released. It could be more painful than you thought.