Deadline for paying back 2/3's of my CARES act distribution

[Tom Degroodt]

I took a 100K distribution from my 401K thru the CARES Act and split the taxes for 3 years.  I paid taxes in 2020 on $33,333.33.  I would like to pay back $66,666.66 so that I can have an amended return filed for 2020 and not pay the taxes due on the 2nd installment for 2021.  I am having a hard time finding whether I have to pay the $66,666.66 back before the end of 2021 calendar year, or just before filing my 2021 taxes.  Any help would be apprciated.