Change password for .pst file

A user pst file is asking for a password even do he did not give it a password. I known this for a fact because my T/s personal set all user pst file w/password. All the user did is switch one PC to another PC. Yes he log completed off the when he log back-in and re-map to his pst file is asking him for a password. How do I reset the pst file for not asking for a password or do I have to use last week backup.