If you used the Online version the only thing you can do with 2018 is download the pdf or .tax2018 file to your computer. You can't delete the amended return or get the original.
You can download the .tax2018 file and open it in the 2018 Desktop program. Then you can undo the amended return and put it back to the original return. Then you can use it to transfer into a new 2019 return.
Post back if you want to do anything with 2018.
