How do you download an amended 1099 from my broker and override the previous one electronically?

Hi @dalemorrison894- First, you'll need to delete the original 1099-B by using the instructions included in the TurboTax FAQ I'm linking below.

How do I view and delete forms in TurboTax Online? 

Then import the corrected information into your TT program just as you did the original.  Just in case you need it, here's a link to the FAQ with that info.

How do I import my 1099? 

Many (or most) financial institutions do not make corrected forms available for import - they must be manually entered.    Often when only a few things change it is easier to just edit the prior form and make the change rather then re-entering the entire form and then check the "corrected box".

I agree with macuser_22, however instructions if there are substantial changes.

You need to delete the original downloaded broker information, then download the updated version again.

Here's the general procedure for viewing the forms list and deleting unwanted forms, schedules, and worksheets in TurboTax Online:

1. Open or continue your return in TurboTax.
2. In the left menu, select Tax Tools and then Tools.
3. In the pop-up window Tool Center, choose Delete a form.
4. Select Delete next to the form/schedule/worksheet in the list and follow the instructions.