I'm sorry, but the only way to downgrade your product is by using the "Clear and Start Over" feature.  Just in case you're unaware of the instructions for using that feature, I'm linking it below.

How do I clear and start over in TurboTax Online? 

If you want to use a lower version without losing your entries,  one thing you can do is move your Online file to a lower Desktop program and continue without paying for Online or needing to reenter your info. All the Desktop programs have the same forms.  You just get more help and guidance in the higher versions.

To continue in the desktop version see this…….

https://ttlc.intuit.com/community/choosing-a-product/help/how-do-i-switch-from-turbotax-online-to-th...