You didn't answer the questions I asked above. So I need to ask again.
Have you already filed your 2018 return, or is it still in preparation? What you can do depends on the answer to that.
If you've already filed your 2018 return, you can't downgrade it yourself. You'll need to phone TurboTax Support to tell them you upgraded too far and ask them to unlock any prior returns. They might not even charge you. (see contact info below).
If you have not yet filed your 2018 return, we can tell you how you may be able to downgrade to Free Edition by using "Clear and Start Over", then upgrade to PLUS and pay the 19.99 by credit/debit card.
Are you wanting PLUS to unlock prior online returns, and are your prior returns in the same account as the 2018 return?
As I mentioned above, if you've already filed 2018, you'll have to phone TurboTax Support to resolve this.
Hours are 5AM-5PM Pacific (8AM-8PM Eastern) Monday-Friday.
FAQ: What is the TurboTax phone number?
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632
If you only need the 2017 AGI, there is a free option. When you look in "Your Returns & Documents" and choose 2017, there should be a free link "View adjusted gross income (AGI.)"
There's also a free option if a prior-year tax return transcript will suit your purpose. You may be able to immediately download one at the IRS website if you qualify to open an IRS account there. Otherwise, to order a transcript to come by postal mail takes 5-10 calendar days.
http://www.irs.gov/Individuals/Get-Transcript
