First of all, sorry you want to leave us! If this is because of problems you've had with the app, please contact firstname.lastname@example.org and we'll do our best to resolve your issues.
If you're sure you want to cancel your subscription then please be aware that doing so will not refund the balance of any time remaining on the subscription, it simply prevents the subscription from automatically renewing on the renewal date. You will also retain access to any subscription only features and content until the subscription period expires.
Open the Settings app, then tap on your profile at the top. Next, tap on 'iTunes & App Stores', then tap your on Apple ID at the top. Finally, scroll down to the 'Subscriptions' section to cancel your Zombies, Run! subscription.
Open the Play Store app and tap the left Menu icon. You should see a tab labelled 'Subscriptions' there where you can select your Zombies, Run! subscription and cancel.