I amended my 2019 return to add a late arriving k-1, and filed. Now I need to amend my 2020 return to reflect this change. Which topic's "Update" do I click, to do this?

After you start the amendment process you can click on ANY of the options as it will simply allow you back into the return then you can direct yourself back to the section you need to change. 

Thanks for the response. Let me try this tomorrow and confirm.

Yes, that seems to work like you said. However, for any topic I pick it is asking for information afresh; at which point would it ask me to load 2020's original return, so that I can amend it?

You should already be in your 2020 original return.  You start amending by opening your 2020 return.  Then you click on amend and make the changes.  

I loaded my 2020 return, but I couldn't find the "amend" button. Can you please tell me where that is?

It's under Federal Taxes Tab or Personal (Home & Business)
Other Tax Situations
Other Tax Forms
Amend a Return - Click Start or Update