Revised Bill to be issued on change of HSN Code?

sir , my client has made sale bill in march 2020, april 2020 and may 2020 with hsn code 3004 and charge gst @ 12% while department says it is covered in hsn code 3808 and gst rate is 18% , now management decide to change it with hsn 3808 with gst 18%, now question arises how to shows it in books , whether we have to revise the sale bills with correct hsn and gst rate or we have to issue debit note of 6%, please tell

Note:-gst return already filed for the same month