How do I cancel my HBO GO subscription in iTunes?

Your HBO GO subscription auto-renews at the beginning of your billing cycle and is managed through your iTunes account. There are two ways to cancel your suscription to HBO GO, one is directly from the HBO GO App and, the other one is from iTunes:

From HBO GO:

In the HBO GO APP go to Settings
Go to the Subscription tab and tap on "Cancel"
Keep in mind that in this moment you are leaving the HBO GO environment and being redirected to iTunes. Confirm you want to continue.
While in the iTunes environmente, go to "Subscriptions" and tap on HBO GO
Choose Cancel Subscription
Please make sure you complete steps 4 and 5 so your cancellation process is complete. Case you don't go through these steps, your subscription will continue active and will generate charges on your payment method.

From your iPhone or iPad via iTunes:

Open your device Settings, and then tap iTunes & App Store
Tap your Apple ID at the top
Tap View Apple ID and sign in if prompted
Tap Subscriptions
Find and tap your HBO GO subscription. If you don't see an HBO GO subscription, make sure that you're signed in with the correct Apple ID
Choose Cancel Subscription or turn off Automatic Renewal