In order to downgrade your account, you would need to clear your return and start over from scratch, unfortunately. However, if you have added add-ons to your account, you can follow the instructions in the following FAQ to remove those.
https://ttlc.intuit.com/replies/3288495
Please note: You are able to begin your tax return using TurboTax Free, but if you do not qualify for TurboTax Free, you will be prompted to upgrade in order to continue and file your return.