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.
