When you posted your question you selected TurboTax Premier Online from the list indicating what product you are using. See the product indicator to the right of your screen name at the top of the question. But your description of trying to override form 8949 sounds like you are using the CD/Download TurboTax software.
You cannot make the correction directly on Form 8949. Furthermore, you don't just change the basis that was reported on your Form 1099-B or equivalent statement from the financial institution. You have to enter the correction as an adjustment. The Step-by-Step interview in TurboTax Premier handles this situation easily. First remove any overrides that you made. This is very important. Then go back to Step-by-Step mode and edit the transaction in question. On the screen where you entered the details of the sale from the 1099-B, click the button that says "I'll enter additional info on my own." On the next screen, enter the correct cost basis in the box labeled "Corrected cost basis." TurboTax will make the appropriate adjustment entries on Form 8949.
If the button isn't on the screen with the details of the sale, it may be because you already clicked that button previously. In that case, just click Continue and the next screen will have the box for "Corrected cost basis."
