If you've under-reported an invoice value in your GSTR-1 and GSTR-3B for a previous month, you can rectify this by amending the invoice in your current month's GSTR-1 filing. The additional tax due on the corrected amount will need to be paid along with applicable interest in your GSTR-3B return.
24 June 2021
It can be rectified by showing the Amendment to invoices in GSTR 1 of May month. The tax on remaining amount would be payable along with the interest.