Get your taxes done using TurboTax

I think you mean you downloaded your Form 1099-B into TurboTax but you need to adjust the cost basis on some of the investment sales. If so, you don't need to enter everything from scratch, you just need to edit those entries on which you need to adjust the cost basis. You can do this by entering into the Investment Income section and editing the entries in the Stock, Cryptocurrency, Mutual Funds, Bonds, other section.  

On the screen that says Review this (name of investment) qualifying sale check the box that says the cost basis is incorrect or missing on my form 1099-B. On another screen you will see an option to enter the correct cost basis.