Since you have already filed the 2018 return and have now upgraded too far to Deluxe, you cannot downgrade it yourself. If you try to pay now, it will charge you 59.99 for Deluxe, and possibly 44.99 for a state return. But this can be resolved.
You'll need to phone TurboTax Support during business hours, tell them you accidentally upgraded too far after filing, and ask them to unlock the prior returns for you. They may not even charge you.
Hours are 5AM-5PM Pacific (8AM-8PM Eastern) Monday-Friday.
FAQ: What is the TurboTax phone number?
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632