If you decide to cancel your FrostWire ads free subscription just follow these steps:
1) Make sure you are properly logged with the Google Play account you used to purchase the plan, If yes go to your Play Store > Account > Subscriptions to see your plan and click the option 'Manage':
2) On 'Manage Subscription' select the option 'Cancel subscription' and then confirm this action. You should receive an email from Google Play confirming the subscription was actually canceled.
If you would like to ask for a refund please check the email you received from Google Play with the possible options they offer to ask for a refund once you cancel your subscription.