Cancellation is only available for the monthly subscription since all other plans (3,6,12 months) are non-recurring plans; they are non-cancellable.,
In case of a monthly subscription - You can cancel your membership anytime you like. You will continue to enjoy all Premium benefits till the remainder period of your subscription, post which your account will be switched back to basic, and you won't be charged any further.
Please visit this article to know how to cancel your Pro membership.