Please clarify what you mean by "filed online."
Are you saying you prepared the 2018 return in Online TurboTax instead of desktop software?
If you prepared the 2018 return online, then you need to save the online 2018 return as a tax data file for the software to use. So go back and save it as a *.tax2018 data file to be able to open it in desktop software.
If it contains a state return, then you'll need to download the state program into your 2018 desktop software before opening the data file. You may have to start a new dummy return just long enough to download the state program.
If you were due a Federal refund, you should wait on the refund to arrive first before amending.
The amended return cannot be efiled; it has to be printed, signed, and mailed. It is recommended to mail by a method that has tracking such as certified return receipt or priority mail, etc.
If you amend the Federal return, you'll likely have to amend the state return as well.