The 2018 online program is closed for the switch to the 2019 online program so you can't upgrade at this time. If the prior year return is locked you must either wait for the 2019 program to be live sometime in December OR use one of the IRS options listed below.
To access your current or prior year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on the Year and Click on Download/print return (PDF)
You can also get free transcripts of your federal return from the IRS - http://www.irs.gov/Individuals/Get-Transcript
For a fee of $50 you can get a complete federal tax return from the IRS by completing Form 4506 - http://www.irs.gov/pub/irs-pdf/f4506.pdf
@Byronmoore On December 4 -- tomorrow-- the transition from 2018 to 2019 software will be complete and you will be able to purchase PLUS and get to your old returns.
Access to prior year returns
You can get a free transcript from the IRS:
How do I get a copy of a return I filed using online TurboTax?
How do I get a copy of a return I prepared using desktop TurboTax?
Note: Every year, before mid-October, print a copy of your current year tax return and save it as a tax file and pdf, too. That way you will have it when you need it for school, a loan, next year’s tax return, etc. without going through a lot of aggravation (and expense) to get it.
@Pablo67 Read the response directly above your own post. It applies to you.
Scroll up and read the whole thread that you posted to if you want an answer.