App Store
How do I change the email address that I used to set up my App Store?
iPad Air Wi-Fi
Question marked as
See the instructions in: Change your Apple ID - Apple Support
and be sure to note the following from that link:
Before you change your Apple ID email address, you need to sign out of every Apple service and device that uses your Apple ID. If you want to use your iPhone, iPad, or iPod touch to change your Apple ID, stay signed in to your Apple ID on just that device.