quart regarding debit note

I have collected tax @ 12% instead of 18% in April and later on I have issued in June 6% debit note to customer then how to show in GSTR 1

problem is that there is no rate of 6% in Debit/credit note column and there was taxable amount same as earlier bill so that I have issued only tax amount of debit note

so how to show in GSTR 1