Debit note pertaining to sales only should have been reported in GSTR 1. Nothing can be done in the new gstr 3B system. next month you will have update credit notes for the same and then that would nullify the debit notes.
Ideally you should not upload debit note for purchases in GSTR-1. Now since you have uploaded it so there are two options either amend it in the next month with values as 0 or issue a credit note against this debit note in next month