After you file

This answer assumes your 2017 returns and completed 2018 Free Edition return are in the same account.

Here's how to open a completed 2018 return in order to upgrade and pay for PLUS.  If you have already done the upgrade steps, then after opening the return back up you can skip to the payment steps

NOTE:  If you have already filed the 2018 return, then do NOT make any changes in the return.

• Log in and go to the section "Your Tax Returns & Documents".  Click on that line or the word SHOW to expand that section.
• Choose 2018.
• Then click the link "Add a State".  (you aren't really adding one; that just opens the return back up.)
• Once the return is open, if you have not yet upgraded, then click in the left menu column on Upgrade.
• At the Upgrade screen choose PLUS.  NOTE:  If PLUS is grayed out, that means you are already in PLUS.  In that case, do NOT upgrade any farther, or you'll have problems.
• If you are already in PLUS, then you just need to pay for it.
• In the left menu column, click on TAX TOOLS, then choose PRINT CENTER.
• In the Print Center choose "Print, save, view THIS YEAR's return."   (yes, you say "this year's" because you are paying for PLUS 2018 to unlock the prior returns.)  You don't really have to print it.
  
• On the next screen choose to PAY NOW.  (for this unlocking purpose, you cannot use the "Pay Later" feature if it's offered.)
• The next screen will be an Order Summary.  It should show PLUS for 19.99.
• Next screen says "Pay with Credit Card", but it can also be a debit card or prepaid debit card.