If your password is expired then just connect and SQL*Developer will prompt for your new password. If you want to change your password while it's not expired connect and use the following statement as normal user:
ALTER USER <your_name>
IDENTIFIED BY <your_new_password> REPLACE <your_old_password>;
Sorry! This has been my fault. Please kindly replace <username> with the name of your database user <Newpassword> with your new password and <Oldpassword> with your old password. The ORA-01935 simply means that Oracle does not know about a user named <username>.
ALTER USER hr
IDENTIFIED BY newhr REPLACE hr;
I hope that this will bring the desired success to you.
I tried whatever you told me before your last message, of course i didn't type <username> :) But i couldn't change it at home maybe because of restricted privilege since we're using the accounts of our university...