Clarification in GSTR 1

Sir,

Good morning.

I require one clarification by you :

• On change of financial year from 201920 to 2020-21, previous bill series continued in 2020-21.
• Means, if bill. no. 100/201920 pertains to last bill of 2019-20, another invoice number in next FY 2020-21 had been 101/2019-20.
• Please advise do I require and modification in GSTR1 or let it continue in 202021.
• Means invoice series for block of two years.
• If it needs rectification, how can I do it and take back the invoice from buyer, Regards

Regards