Are you absolutely, positively satisfied that they did not get the recovery rebate? https://www.irs.gov/newsroom/2020-recovery-rebate-credit-topic-g-correcting-issues-after-the-2020-ta...

Follow https://proconnect.intuit.com/support/en-us/help-article/federal-taxes/amend-individual-return-1040x... for amending

Remove the check mark in the 1040X section, then be sure to enter the amount they should have received in the Recovery Rebate section and see it that then matches the as filed. Then go to the 1040X screen to start the process by checking the box. Enter -1 as the actual amount of recovery rebate and give the wording in the 1040x that it is the reason for filing.