How do i upgrade to plus to access my 2017 return after i have already filed 2018?

[mesquitebean]

Are your 2017 and 2018 returns in the same account, and is your 2018 return currently in Live Self-Employed edition? (that's what is indicated under your question above.)   That's the highest of all the editions.   Did you use Live Self-Employed Edition to file your 2018 return?  If so, the 2017 return should not be locked if it's in the same account.

Or did you use Free Edition for 2018, and have now upgraded way beyond PLUS, and can't downgrade to PLUS since you have already filed 2018?

If you have upgraded too far after filing, you'll need to phone TurboTax Support tomorrow, tell them what happened, and ask them to unlock the 2017 return.  They may not even charge you.

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

[mesquitebean]

The above comment tells you how to ask TurboTax Support to unlock your 2017 return.   There's another option if a tax return transcript will suit your purpose.   You may be able to immediately download a free return transcript 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