You have to sign onto the 2018 account using the exact same User ID you used to create the prior year online tax return.  Enter your personal information in the 2018 Free edition. On the left side of the screen click on Upgrade.  Scroll down and select PLUS More Help & Tools.  Click on Upgrade Now (if Plus is grayed out then you are already using Plus and only need to pay for the upgrade)

To pay the TurboTax online account fees by credit card so you can access your prior year online tax returns, click on Tax Tools on the left side of the online program screen.  Then click on Print Center.  Then click on Print, save or preview this year's return.  On the next page, to pay by credit card, click Pay Now.  On the next screen just click on the Continue button.  The next screen will show a review of your Fees, click on Continue.  Complete the next screen to pay by credit card and click Continue.