Online is closed right now to get ready for 2019. You can upgrade in December when Online reopens.
Or One user posted what he did to pay for Plus now to unlock prior years. So try this.......
"After signing in, I clicked "Help" in the top right corner.
On the right side of the screen, a window opened up with a search bar, FAQ's, and at the bottom "Contact us"
I clicked "Contact us" and it said that I needed to purchase Plus to be able to talk.
I believe "purchase" or "Plus" was a hyperlink which I clicked and was taken to a page to confirm my payment info. After I did that, I had access to my previous year tax returns. "
That might only work during business hours M-F 5-5pm Pacific time (8-8 Eastern). And we don't know if that will give you Plus for just 2018 or also 2019.
