To downgrade your Synthesia plan:
Navigate to Settings and navigate to Billing.
Select the Downgrade, at the top right of the window. All the plan options will be displayed.
Choose the new plan you want.
Click Downgrade.
Notes:
Downgrades are effective at the end of the ongoing billing period and therefore have no proration.
A scheduled downgrade can be cancelled from Settings > Billing , using the Cancel downgrade option as shown below.