If you filed a 2020 tax return reporting your unemployment compensation but before the exclusion was available, then do not amend your 2020 tax return.
The IRS will apply the exclusion and refund any amount based on the changes to your tax return. Go to this IRS website for information - https://www.irs.gov/newsroom/irs-to-recalculate-taxes-on-unemployment-benefits-refunds-to-start-in-may