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.