I had made a supply in August. In September, the recipient decided to permit increased rate for the supply. How will I declare the debit note to the tax authorities

Debit note should be issued with immediate effect, and the details should be declared in the return of outward supplies for the month of September.

(ICAI FAQ PUBLICATIONS 06-09-2017 TAX INVOICE, CREDIT AND DEBIT NOTES: FAQ NO. 28)