How can I enter information from a prior return that wasn't done in TurboTax so I can compare prior year to this year's TurboTax return?

You missed the chance to do that.   You can only transfer 2019 data at the very beginning before you start to enter any new 2020 information.   Once you start entering 2020 data you cannot go back and transfer 2019 in---unless you clear and start over from the very beginning.

How to transfer last year’s return? https://ttlc.intuit.com/questions/1900883

https://ttlc.intuit.com/questions/1900883-how-do-i-transfer-last-year-s-return-to-turbotax-online

https://ttlc.intuit.com/questions/2262857-when-transferring-a-tax-file-what-information-carries-over-to-my-return

Yes, you can still update this by going to the section to upload the PDF. However, to add this in the TurboTax Online account will cause the 2020 tax return to be cleared and you'd have to start over, as @xmasbaby0 said.

To do this in TurboTax Online, follow these steps:

• From the left menu, select Tax Tools.
• Select Tools.
• At the Tools Center pop-up window, scroll down to Transfer last year's TurboTax return from your computer.

For more information, see: How do I transfer last year's return into TurboTax Online?

Keep in mind if you are using the TurboTax CD/Download, follow these steps:

• Go to Forms Mode.
• Double-click on Open Form
• Enter what-if in the search box.
• Highlight What-if Worksheet and click Open Form.
• It will be 2020 rates, but you can enter the 2019 details which would provide a comparison to 2020 without having to clear and start over. 

@fouryuks-hotmail