based on http://scratching.psybermonkey.net/2009/06/postgresql-how-to-reset-user-name.html

1. Edit the file sudo nano /etc/postgresql/9.1/main/pg_hba.conf
2. change into local all postgres md5 local all postgres trust|
3. then do a restart sudo service postgresql restart

4. now enter into the PostgreSQL using

   psql -U postgres
   ALTER USER postgres with password 'new password';
   

5. then again change the pg_hba.conf as earlier and restart the PostgreSQL for invoking the password protection