Bill Cancellation

Few facts needs to be understood before taking the decision.

Is the date of invoice and date of cancellation in same month?
Is there a movement of a single goods before the deal was cancelled?
Its recommended to issue a credit note instead of cancelling invoice for the sake of management of accounts.

However, if there is no movement of goods as well as there is no receipt of payment and where you expect that the deal is not gonna crack on a condition that everything falls in same month then I suppose one can also cancel the bill.

Bill is of the month of November 2020

You should issue credit note or cancel the bill

Since Gstr-1 already issued the best way is to issue a credit note.
You can also amend the invoice as the second alternative

Issue credit note and show in gstr1