Log back into TurboTax and follow the steps in this link to get back to your investment sales section of the program.
Once you get to the investment section, you will select edit using the little pencil icon to the right of the applicable investment sale.
This will take you to the details for that specific stock sale. Scroll down on the page asking you review the sale and go to the section titled sales info. You will make your adjustments to the right of the input for Box 1e - cost or other basis.