Gstr 1 modification

Good day,

while filing gstr1 qurarterly retrn (march 2019 ) for my client, an export  invoice had been worngly booked as it was only a performa invoice and not  an tax invoice.

kindly guide me how to delete that in gstr1of march 2019.. pls clarify whether, only after modifying gstr1 of march 2019,,the gstr1 of this quarter shd be filed.(apr-jun 2019)

is there any option to modify in march 2019 gstr3b.

pls guide.tnx