Hi Anil,
the same thing happened to me a few days ago when I installed 1.52. Somehow the 'permissions' for the config.php file got changed from 644 to 640. If this is what happened to you also, then read on ....
If you are using 'shared' hosting, you should contact your hosting company and ask them (they are the only ones who will be able to do this) to change the permissions of this particular file to 644. When 644 (or indeed 755 or 777 etc.) you will then have access to updating the file.
If using a dedicated / your own
server, you (or the root / admin) can do similarly.
Kindest Regards,
Seaghan.