If you have no need to file a SCH C (usually because you have a K-1 from the S-corp or partnership) then you have to start over in order to downgrade. You can't do this if you have already paid your fees.
With the return open so you can edit it, click My Account at the top right and select the clear and start over option. A few screens later you can select the version of TT you desire to start with.