You may deactivate auto-renewal for your subscription from the Knawat dashboard as follows :
1-Select the desired store you need to deactivate the auto-renewed subscription for.
2-Click on Store settings > Billing > Payment methods
3- Delete your payment method by clicking on the Bin icon next to your credit card.
You will no longer be charged when your next billing invoice is due and your subscription will be automatically downgraded to free.