I need to enter a W-2c because I had to return a bonus from a previous job. How do I make this change and report my lower tax liability?

[Mike9241]

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. 

[rjs]

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.