Where do I enter the correct/adjusted cost basis for stock sold from my ESPP?

When you enter your 1099-B, enter the reported basis as it is on the 1099-B.  There will be a box to check if the basis reported on your 1099-B is missing or incorrect, which is very likely.  TurboTax will walk you through getting it reported correctly.  

in the TurboTax 2019 Home and Business Edition, I do not see a checkbox to adjust

Look for the Guide Me Step by Step button and you will be able to enter.

@ckoontz50

The "I'll enter additional info on my own" is not showing on Mac 2020. Any idea where it went?

You can adjust the cost basis box. As long as the correct information is transferred to the 8949, and sch D, cost basis and sales price, that is the concern.

@neroburner

Q: After I enter the cost basis in 1e and "check" the box what Adjustment Code do I select. B or E?

With the sale of ESPP stock you have 2 ways of entering the sale:

1. You enter all the information on the "Tell us about your [Broker Name] 1099-B" page, entering the 1099-B exactly as it reads, wrong basis and all, and then click the "I'll enter additional information on my own" button and on the next page enter the correct basis.
2. You enter all the information on the "Tell us about your [Broker Name] 1099-B" page, entering the 1099-B exactly as it reads, wrong basis and all, and then click the "Guide me step-by-step" button, telling TurboTax that you're entering the sale of stock received via an ESPP, and completing the interview.

Either method, properly done, should result in a correctly entered sale.  The first method puts a "B" in column (f) of the Form 8949 while the second method puts an "O" in column (f). 

1. You received a Form 1099-B (or substitute statement) and the basis shown in box 1e is incorrect...
2. If this transaction is reported on a Part I with box A checked at the top or if this transaction is reported on a Part II with box D checked at the top, enter the basis shown on Form 1099-B (or substitute statement) in column (e), even though that basis is incorrect. Correct the error by entering an adjustment in column (g)

As I am amending my 2020 tax return, I find it so confusing about how/where to enter adjusted cost basis. I looked back and forth on different screens, but couldn't find a place clearly labeled for adjusted cost basis. It's frustrating. Can someone please show the screen and selection for entering adjusted cost basis? I'd really appreciate it.

Since you are on the downloaded version switch to the FORMS mode ... go to the 8949 and make the adjustment directly on the form. 

@Critter-3  Thanks a lot for help! I guess I have to enter each sale individually on Form 8949 worksheet, which is a little tedious. Really wish Turbo Tax could make it easier for users to enter such information by using its interface with proper instruction.