The only way to downgrade your online product is to use the "Clear and Start Over" feature. Keep in mind that all of your data entry will be deleted. Instructions are below.
https://ttlc.intuit.com/replies/3288495
*** 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.***