which year did you return the bonus? in the year received or in a future year. different rules apply depending on which is your situation.
if the same year filing an amended return with the corrected numbers - as if you never got the bonus is correct.
if in a following year and the repayment was more than $3,000 you can take as a deduction on schedule A line 16 denote as IRC 1341 repayment
OR
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 12d denote as IRC 1341
no amended return is required if all that was properly changed were social security and medicare amounts.
