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.
Did you repay the bonus in the same year that you received it, or in a later year? If you repaid it in a later year, the employer should not have changed the box 1 amount on the W-2c. However, if that's what they did, rather than fight with the employer, I would just report the W-2c, whether or not it's correct.
You don't "enter a W-2c." A W-2c is a correction to the original W-2. To amend your tax return you change the original W-2 entries to reflect the corrected amounts shown on the W-2c. You have to amend your tax return for the year that the W-2c is for. Look carefully at the W-2c to see what year it is for. On a standard W-2c the year should be in box c.
If the W-2c changes only boxes 3 through 7, and not box 1, you do not file an amended return unless you had Additional Medicare Tax (Form 8959) in your tax return for the year that the W-2c is for.