Amend prior return to include recharacterization?

I contributed to a Roth IRA in 2020, but then realized in Jan 2021 that I was not eligible due to income limit.  I did a recharacterization and transferred the contributions from the Roth into an traditional IRA.  When I filed my tax return for 2020, I did not include the recharacterization.  I would like to go back and correct this, but I do not know which year to amend.  I recieved the 1099-R and 5498 showing the recharacterization that was done in Jan 2021 for tax year 2020.  Which year do I amend, 2020 or 2021?