In absence of GSTIN , I have file the details in B2C form of gstr 1 in June 2018. But now I have GSTIN of the client and want to amend the returns so that he can avail the input credit.
No there is no provision to rectify any thing in return once filed , so Rectify your mistake in July GSTR1 by adding that bill with its original date & under ammendment table B2C reduce the value of sale equal to that bill