"However, the Turbotax 2016 amend software doesn't show me an option to manually enter the new cost basis information."

Sure it does.  I assume that you entered the sale using the RSU step by step process, gave TurboTax the per share fair market value at the vesting date, which became your per share basis, and then sometime between vesting and sale got a return of capital distribution.

You have 2 ways of dealing with this.

The first is to delete that sale and re-enter it using the default TurboTax 1099-B entry form.  You enter the 1099-B exactly as it reads, wrong basis and all, but then click on the "I'll enter additional info on my own " blue button.  On the next page enter the correct basis in the "Corrected cost basis" box.

The second way is to go back to the RSU step by step interview and in the per share fair market value enter a per share number that reflects the subsequent return of capital.  That is if the original FMV per share was $10 and the return of capital, on a per share basis was $.10 you'd simply enter $9.90 in the box.

Either way should get you to the correct answer.

Tom Young