You do not have to file an amended return. As of today, 3/23/21, the IRS posted that those who have already filed do not need to file an amended return to back out unemployment benefits.
Per the IRS, if you have already filed your 2020 Form 1040 or 1040-SR, there is no need to file an amended return (Form 1040-X) to figure the amount of unemployment compensation to exclude. The IRS will refigure your taxes using the excluded unemployment compensation amount and adjust your account accordingly. The IRS will send any refund amount directly to you.