If you subscribed to Playster through the App Store, your membership is being handled by Apple - not Playster.
To cancel your subscription on any Apple device, please follow these instructions.
If you are on a paid membership, you’ll be able to continue using your account until your next billing date (you will not be charged again). If you are still in your free trial period, you will lose access instantly.
If you're experiencing any issues processing your cancellation through Apple, feel free to contact us.