When you enter the sale in TurboTax enter the correct acquisition date, even if there is an incorrect date on the 1099-B. After you enter the sale, the next screen will say "Let us know if any of these situations apply to this sale." Check the box that says "I need to correct the holding period reported on my 1099-B." TurboTax will tell you that it has been updated to long-term. That's all you have to do. The sale will be reported with adjustment code T on Form 8949, indicating that the holding period has been corrected.