There is no way at all to work on a 2018 return online. And you say you used desktop software to prepare that return, Did you uninstall the software and throw away the CD? (Why?)
If you need to amend that return it must be done using 2018 desktop software. Do you have the tax file saved in your hard drive? Returns prepared using desktop (CD/download) software are not stored online--they are only stored on your own hard drive.
See if customer support will provide a download of the 2018 software for you so that you can amend.
To call TurboTax customer support
https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number
They are available from 5 a.m. to 5 p.m. Monday - Friday Pacific time.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
