This is mainly a user community forum. I don't work for TurboTax but can guide you with better steps than the ones TurboTax provides. I admit it's a non-intuitive and frustrating method, and we have complained about it to them before to no avail. To give you proper steps I have to ask you some questions first.
Do you find the 2017 online return OK, but when you try to download, does it ask you to pay? If it was prepared in Free Edition last year, access will be locked. If that's your situation, we can tell you how to pay to unlock your 2017 return. The method to upgrade and unlock it depends on what all you have done with your 2018 return.
If you find the 2017 return, but access is locked, please answer all of the following:
We can give you precise steps to upgrade and pay to unlock a return once you answer all the questions above.
There's also a free option if a 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.