You really should not be mucking around in the forms comprising your income tax return. Doing so can nullify TurboTax's "accuracy guarantee" and, in any case, what you are trying to accomplish can be done through the interview itself.
In the desktop version of TurboTax, (having never used the online version my assumption is that the desktop and online versions largely mimic one another), I'd enter the 1099-B exactly as it reads, wrong basis and all. (Another assumption - the wrong cost basis is what's being reported on the 1099-B and the basis is being reported to the IRS.) Then I'd click the blue "I'll enter additional info on my own" button and on the new screen that shows up when I do that I'd simply enter the correct basis in the "Corrected cost basis" box. That "correct" basis then flows to Form 8949.
Of course if the cost basis on the 1099-B is wrong and the basis isn't being reported to the IRS, you're free to simply enter the basis you believe is correct when you enter the 1099-B.
Tom Young
