Should I amend my return if I filled before the cares act?

The CARES Act was signed into law a year ago, on March 27, 2020. You could not have filed your 2020 tax return before then. Are you talking about your 2019 tax return? Or do you mean that you filed your 2020 tax return before the American Rescue Plan Act was passed a couple of weeks ago? If you are talking about the ARP Act, what specific provision do you think you would have to amend for? If you are thinking of the unemployment exclusion or the suspension of Premium Tax Credit repayments, do not amend your return. Wait for the IRS to issue instructions about what to do.