Change password of an user

Check the user status and existence

select username, account_status from dba_users where username = 'SCOTT';

Change the Password of an User

ALTER USER SCOTT identified by NEW_PWD;

You can also lock and unlock the user if its show locked in Account_status

-- Unlock/lock a user
ALTER USER SCOTT account unlock;
ALTER USER SCOTT account lock;

If user can change password on first login attempt

ALTER USER SCOTT password expire;