As per Sec 34 of CGST act
where one or more tax invoices have been issued for supply of any goods or services or both and the taxable value or tax charged in that tax invoice is found to exceed the taxable value or tax payable in respect of such supply, or where the goods supplied are returned by the recipient, or where goods or services or both supplied are found to be deficient, the registered person, who has supplied such goods or services or both, may issue to the Recipient one or more credit notes for supplies made in a financial year containing such particulars as may be prescribed
As Per Sec 34 (2) of CGST Act
Any registered person who issues a credit note in relation to a supply of goods or services or both shall declare the details of such credit note in the return for the month during which such credit note has been issued but not later than September following the end of the financial year in which such supply was made, or the date of furnishing of the relevant annual return, whichever is earlier, and the tax liability shall be adjusted in such manner as may be prescribed:
Provided that no reduction in output tax liability of the supplier shall be permitted, if the incidence of tax and interest on such supply has been passed on to any other person.