Had my taxes done with a preparer who made MASSIVE grossly negligent errors. How can I file an amended return using the software as he thinks he did nothing wrong?

[VolvoGirl]

To amend a return you did somewhere else or you don't have the original.tax file,  you would first have to fill out Turbo Tax like you are doing the original return and pay for it.  You have to recreate the actual wrong original return.  Then when it matches each line that you really filed you have to tell Turbo Tax you will mail it so it thinks it was filed.  Then you can amend it.

For the current year you can use the online version by first recreating the original return exactly as filed.  Then pay the fees and say to mail it and print it out so it think you filed it.  Then when it matches every line you can amend.  How to amend an Online return   https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577

Or you can buy the Desktop CD/Download program here, https://turbotax.intuit.com/personal-taxes/cd-download/

[fanfare]

You must have a copy of the original  massively incorrect tax return you want to update (or a transcript).

The simplest way is to prepare your correct tax return (using TurboTax for example, or other means) and print it.
Then lay the paper printouts of both returns (correct and original) side-by-side.
You now have all the information needed to prepare Form 1040X by hand.
It's pretty straight-forward.

The blank form in fillable PDF format and its instructions are available at www.irs.gov.
Mail to the same address you would mail your original tax return.