If you are using TurboTax Online, you can make a copy of your last year's tax return and enter this year's information.  Once you open TurboTax Online and sign into your account under the return you want review click on Copy- and work in that specific return.

Thank you for choosing TurboTax.

I have opened my TurboTax account, and opened the list of past years, but cannot find any copy button.  I am using TurboTax Online. Please provide instructions.

Thanks, Tom

Once you sign into your TurboTax Online Account, you will see the list of returns that you have worked on or filed. The screenshot below will show you where to find "COPY."

Thank you for choosing TurboTax.

I only see Rename.  The Copy is missing.

Unfortunately, if a copy wasn't made before filing you will need to use your copy of your saved PDF to complete a new return.