I'm having the same issue.  Just wants me to exit to IRS.gov.  I just want to pay the upgrade and finish filing.  Surprised nobody has answered this question!  What gives?

during the process of filing you will receive a prompt to upgrade which version you are using.. just continue filing with the free version until you see this prompt then upgrade and continue filing

What if you have already filed and you want to upgrade?  I don't get the upgrade options in the left-hand menu to choose plus and the phone numbers you get are all automated...no way to reach technical support.

Did you figure this out?  I don't have upgrade in my menu bar on the left, nor do I have tax tools.

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.