B2B sale uploaded in B2C

You can Rectify in Next month GSTR-1 

Yes i explain you what exactly you do in next month 

As per procedure given in GST act up to Now ..Following Procedure For Amendment in B2B  invoices

First you amend the B2C up to amount which you wrongly entered B2B invoices in B2C column in current GSTR-1

after that you entered correct  B2b invoices of September Month in B2B table in current GSTR-1 

That all your B2B invoice correctly entered and B2C amended

(Kindly note that September Month B2B invoices enters in Current Month GSTR-1 with September month date not current month date )