In Android, subscriptions renew automatically.
You need to cancel the subscription in Google Play > My Apps > My Subscriptions.
Or alternatively with a browser by going to Google Play > login > Settings/ My account (upper right hand side) > Order History > More (...) > Cancel subscription
Please check also these instructions if the above aren't helpful:
If you have used a gift voucher to get Premium you don't need to cancel your subscription. It will become free after the paid period and you won't be charged.
The Premium subscriptions are not eligible for refunds.
When you cancel your subscription you will continue having a Premium account until the last day of your subscription period, and then you will be downgraded to a Free account and you will not be charged anymore. You can become Premium again at any time.
Please note that the free trial is valid for 30 days and will continue as paid service from thereon if you don't cancel it. The trial lasts from the exact time you start the trial and 30 days onwards, so please be careful with the time. If you don't want to continue with Premium after the trial period, we recommend cancelling at least a day before, the trial will continue until the end, no matter when you cancel it.