You will need to cancel your current Silhouette Design Store subscription in order to downgrade to a lesser subscription level. Once your subscription is ended, you can subscribe anew to a lower-level subscription option.
Please rest assured that any current subscription credits you have on your account would be carried over to your new lower-level subscription from your previous subscription with their original expiration dates.
NOTE: You may be obligated to pay a cancelation fee if you cancel your subscription before three months have elapsed, subject to the information found under 'Canceling a Subscription' in the site's Terms of Use.