To downgrade your Synthesia plan:
Navigate to Settings and navigate to Billing.
Select the Change plan option on screen. 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.