How do I cancel a pre-ordered book in iBooks?
How do I cancel a pre-ordered book in iBooks from my iPad mini 4?
iPad mini 4 Wi-Fi
You can't on your iPad, you will have to use your computer's iTunes : Account > View My Account (Store > Account on older iTunes), and on your account's page there should be a 'pre-orders' link in the Settings section (the link only shows if you have pre-orders on the account)