The syntax for changing a password is:
alter user user_name identified by new_password;
user_name is the user whose password you wish to change.
new_password is the new password to assign.
For example:
If you wanted to reset the password for a user named smithj, and you wanted to set the new password to autumn, you would run the following command:
alter user smithj identified by autumn;
From http://www.techonthenet.com/oracle/password.php