You don't enter a W-2c separately in TurboTax. When you enter the W-2, for any box that was corrected you enter the corrected amount from the W-2c instead of the amount on the original W-2. If a particular box was corrected twice, enter only the most recent corrected amount. You will end up with only one W-2 in TurboTax, but it will have the final corrected amounts.
