Sorry, You can’t upgrade from the Free File website. You need to start over in regular Turbo Tax here, https://turbotax.intuit.com/personal-taxes/online/
You need to set up a new account and new username and password. But you can use the same email address for 5 accounts. How to start another return in the Online version
If you don't want to start over, we can tell you how to download your *.tax2018 data file from the TurboTax Free File site, and you can then open and continue that file in purchased 2018 desktop software installed on your computer. Otherwise, you'll have to start over in regular Online TurboTax as mentioned above.
For the desktop software your computer would have to meet the minimum system requirements for the desktop software. If it's a Mac: