your options for reporting the repayment in a year different from the year received if it was more than $3,000 then in the year of repayment you can:
a) take as a deduction on schedule A line 16 denote as IRC 1341 repayment OR
b) recompute the taxes in the year you received the payment by excluding it. the decrease in taxes is taken as a credit on schedule 3 line 13b on the return for the year repaid
as to social security and medicare taxes that were not corrected you have to file IRS form 843. you'll need a conforming letter from your employer that it will not claim a credit for you share of those taxes
https://www.irs.gov/site-index-search?search=843&field_pup_historical_1=1&field_pup_historical=1
