If you want to keep things simple, you can just email your service provider or hit them up on live chat to have them cancel your subscription. You can also cancel it manually by finding this option in the app settings.
The VPN services in this article are free and do not require bank details from you, so really, there is no point in canceling the trial. Just let it run its course.
On the other hand, if you had to disclose your payment information to be able to access the free trial, then you might want to cancel your free trial before it elapses, and you are charged for the next subscription.