Mr Ashok Kumar
If u recieved the material physically in the month of nov17 against the purchase bill (sep17),than u book the purchase in nov17 and fill the GSTR2 for the month of sep by click the pending action only for sep 17 purchase invoice and do same things in oct17 GSTR2,but when u fille the nov17 GSTR2 that time u accept the invoice by click accept button.
OR
If u forget(i.e u receive the material in sep17) to take credit in GSTR3B for the month of sep17,than u book the purchase in the month of sep17 itself and u correctly fille the GSTR2 for the month of sep. There is no revise option in GSTR3B .
Thank U