I have an accepted 2019 return that I would like to open to see if I need to pay estimated payments for 2020 with new income assumptions. How do I do this?

What----you want to open your return and mess with it to see about making estimated payments based on "income assumptions?"?  No.  Do not mess with your filed and accepted tax return, or you will make a mess that will keep you from being able to transfer it accurately for your 2020 return next year.    Or if it turns out you need to amend it for some unforeseen reason, you will have a messed up return to work with.  Try the tax caster tool for your income predictions.

https://turbotax.intuit.com/tax-tools/calculators/taxcaster/