GSTR 1 outward supply query

While filing 2nd quarter of GSTR 1 returns we have missed out of July and Aug sales details in GST 1 returns and only sept month sales was taken in the returns. In 3B all three months sales were taken on monthly.

so, how to rectify this mismatch to declare both of month of sale of July &Aug'20 in GSTR1.

Kindly suggest how we can resolve this changes.