Are you using the Desktop program or the online version?  If you don't mind starting over you can transfer from the 2018 Amended return.  If you don't want to do that you should be able to change it now.  What exactly needs changing?

I'm using the desktop version and I amended my 2018 taxes yesterday without much effort.   I found the line that was incorrect and fixed it and my taxes were recalculated and I know how much I owe.   So working on 2019 taxes today and after choosing "Amend" the program gave me an empty form and started asking basic questions which I knew would take at least 30 minutes to get to the issue I had.   It almost feels like the 2018 and 2019 Turbotax programs work in very different ways.

take me to 2019 Federal 2019 1040 taxes

It's common to end up with multiple accounts. First LOG OUT of whatever TurboTax account you're logged into right now. Then use this TurboTax account recovery website to get a list of user ID's for an email address. Run the tool against any email addresses you may have used
https://myturbotax.intuit.com/account-recovery/

If you used the Desktop CD/Download program then the only copy is on your computer and not saved or stored online.

You can also request a transcript from the IRS
https://www.irs.gov/individuals/get-transcript

Belated reply: After choosing "Amend" look for the button "I'll choose what I work on".