"amended 1099"
What kind of 1099? 1099-B? 1099-S? Or some other 1099?
"the capital gain loss is larger."
Is it a larger gain or a larger loss?
Short-term or long-term?
How much of a difference is there?
You can't import or "upload" the amended 1099. You would have to enter the corrections manually.
Did you already file your tax return using the original 1099?
If you have not yet filed your tax return, make note of your refund or payment due, then make the changes to the sales reported on the 1099. You will see how much of a difference it makes. File your tax return using the updated amounts from the corrected 1099.
If you have already filed your tax return, you want to retain the data file for the tax return the way it was when you filed it. Since you are using the CD/Download TurboTax software, open your tax return and use Save As to save a copy of your tax return with a different file name. Now you can experiment on the copy without affecting your original tax return. In the copy, make the changes to the sales reported on the 1099, and see how much of a difference it makes in your tax. If the difference is small you can ignore it. If the difference is significant, you will have to file an amended return. Do not use the copy that you experimented on for the amended return. Make another copy of the original tax return and start the amending process using that copy.
