If you are using TurboTax Free File Program, you cannot upgrade to a paid edition in that same account.
You would either have to start over at the regular Online TurboTax website in a new online account (new User ID), or you can use the following workaround to keep from having to start over.
You can download the *.tax2018 data file from your TurboTax Free File Program by going to the Tax Tools, then Tools.
So first be sure you can download that file. Then you can open the 2018 data file in purchased desktop software (CD/download). You can buy the desktop software directly from TurboTax at this link:
Or you may be able to find it at a reputable authorized online retailer like Amazon, Staples, OfficeDepot, Newegg. Do NOT use an Internet search for a "too good to be true" deal, since there are many pirated software sites or possible malware, ID theft, etc. Only a few big well-known companies like those I mentioned are authorized to sell TurboTax downloads.
NOTE: If your downloaded *.tax2018 data file has a state return in it, then you should download your state program into the desktop software before opening the data file, or you'll get an error message. To do that, you can start a new dummy fake return in the desktop software just long enough to download the state program into the software.
I've spent the last hour going in circles trying to figure out how to upgrade to plus. I followed the directions, which I swear there are 50 different versions out there, but have made zero progress! I need to see my 2016 return for financial aid applications, but I am more confused now then I was when I started!! It's ridiculous!