How to amend 2020 return for retirement plan repayments of Coronavirus distribution

Same way youd amend any 2020 return I suppose...maybe you need to explain the situation more. 

If a 2020 Covid distribution failed to be reported, and now you want to include it on 2020, you'll need to amend 2020, 2021 and 2022 to include that 1/3 amount on each year. 

Seems rather late for IRS to just now be catching up with a missed 2020 distribution, unless this client has been hanging onto the IRS letter for a couple years.

From: https://www.irs.gov/newsroom/coronavirus-related-relief-for-retirement-plans-and-iras-questions-and-answers

"Q7. May I repay a coronavirus-related distribution?

A7. In general, yes, you may repay all or part of the amount of a coronavirus-related distribution to an eligible retirement plan, provided that you complete the repayment within three years after the date that the distribution was received. If you repay a coronavirus-related distribution, the distribution will be treated as though it were repaid in a direct trustee-to-trustee transfer so that you do not owe federal income tax on the distribution.

If, for example, you receive a coronavirus-related distribution in 2020, you choose to include the distribution amount in income over a 3-year period (2020, 2021, and 2022), and you choose to repay the full amount to an eligible retirement plan in 2022, you may file amended federal income tax returns for 2020 and 2021 to claim a refund of the tax attributable to the amount of the distribution that you included in income for those years, and you will not be required to include any amount in income in 2022. See sections 4.D, 4.E, and 4.F of Notice 2005-92 for additional examples."

You'll want to  confirm it's not just three years by tax year, but from the date of the distribution being repaid.