You "may" be able to use the "Clear and Start Over" feature to downgrade your product. Instructions for doing that are included in the TurboTax FAQ linked below.
https://ttlc.intuit.com/community/downgrading/help/can-i-downgrade-to-a-lower-priced-version-of-turbotax-online/01/25591
However, if your 2018 return included a Schedule C business that you need to report closing, you'll need to use the Self Employed program to report it. Just proceed with caution before downgrading your product. Once you clear and start over, all data entry will be deleted.
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