By default, Add-on Prints use the subscription's shipping address. To send them to a different address, follow these steps:
iOS:
Tap "Settings" (gear icon) from your active subscription
Tap Photo Prints
Tap "Shipping address" at the bottom
Select the new address or add a new one
Tap "Done"
Android:
Tap "Settings" (gear icon) from your active subscription
Tap Photo Prints
Scroll down and tap on the "Shipping address"
Select the new address or add a new address by tapping on "+ Add Address"
Tap "Done"