Are you wanting to upgrade to PLUS? What are you trying to accomplish by upgrading? Are you trying to unlock a prior-year return? If so, please answer these questions:
1) Have you already filed a 2017 TurboTax return, or is it still in preparation? And is it in Free Edition?
2) Have you located the prior-year return, and is it in the SAME account as what you are using for 2017? When you try to download/print/view the prior return, does it tell you that you need to pay?
We can tell you how to upgrade and pay when you answer those questions.
Be aware there's a free option if a tax return transcript would suit your purpose. You may be able to immediately download a 2016 return transcript at this IRS website at the link below. If you don't qualify to open a free IRS account there, you can order one to come by postal mail, which takes 5-10 calendar days.
<a rel="nofollow" target="_blank" href="http://www.irs.gov/Individuals/Get-Transcript">http://www.irs.gov/Individuals/Get-Transcript</a>
1?Yes, I have already filled my 2017 turbotax return, and it is free edition.
2) Yes, my prior-year return is in the same account as what I using for 2017. When I try to download prior-year return, it does not tell me to pay.
You said it does not tell you to pay.. Is that a typo? If access to the return is locked, then it should ask you to pay.
This answer assumes your 2016 and already-filed 2017 Free Edition return are in the same account, which you have already confirmed.