If you've already filed your 2018 return using TurboTax, see the instructions in the TT FAQ below:
https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-purchase-plus-after-i-ve-already-f...
If you haven't yet filed for 2018, the instructions below should advise you. Please post back if you have further difficulty.
https://ttlc.intuit.com/community/payments/help/how-do-i-pay-for-plus-in-turbotax-online/00/27001
FYI: You can avoid the extra upgrade to PLUS by saving/printing a copy of the PDF file of your return each and every year. I'm linking those instructions below.
https://ttlc.intuit.com/community/saving/help/how-do-i-save-my-turbotax-online-return-as-a-pdf/01/26...
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***
