See -> View or cancel pre-orders for apps, films, music or books
View or cancel pre-orders for apps, films, music or books – Apple Support (UK)
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What happens when your pre-ordered item is released
When your pre-ordered item becomes available, your payment method is billed and you receive an email. If you turned on automatic downloads, the item is downloaded automatically to the device you used to buy it.
If you can't see a pre-ordered item on your device after it has been released, find the item in your purchases and download it. If the item is an app or game, you can also find it in the App Store and tap or click Get.
If the purchase can't be completed
If a billing issue stops your purchase from being completed when your pre-order is released, you'll get an email. You'll need to update your billing information before you can download the item.
If you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK)