After you file

Thanks for your response! Turns out the problem was the instructions references above say: 

1. Open your TurboTax 2021 software (you must amend your 2021 return in the 2021 version of TurboTax)
2. Select Amend a filed return, located near the bottom of the homepage

The problem is, after one has filed a return the home page never appears again! To get around that, I clicked "Search Topics" and elected 1040x. That opened up the amendment process within my already-filed return, and now the numbers are correct!