Refund of IGST paid for export services

We are export oriented service unit and have filed wrong GSTR 3B for the month of August 2017, instead of putting amount in column 3.1(b)zero rated supplies, we took that in 3.1(a) as normal taxable outward supplies and thus we are unable to apply for IGST paid by us. we have filed GSTR 1 correctly under table 6a. Please guide us with the procedure of refund.