You need the .tax2018 data file for your 2018 tax return in order to amend it. It was on the computer that you used to file the return. If you did not copy the file to your new computer, and you don't have a backup copy, you will have to recreate your original tax return using the 2018 software that you downloaded. You will have to make sure that the recreated return exactly matches the PDF of the return that you filed. Once you have recreated the original return, you will be able to follow the instructions for amending it.
Save a copy of the recreated .tax2018 file with a different file name before you start to amend it, in case anything goes wrong in the amendment process.
Just to make sure you're on the right page, here is a link to the instructions to amend a 2018 tax return that you filed with the CD/Download TurboTax software.
How do I amend my 2018 return in the TurboTax CD/Download software?
