Apple ID
How can I change the primary email that is associated with my Apple ID?
iPhone 8
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
Sign out of all Apple services and devices that use your Apple ID. If you want to use your iPhone, iPad, or iPod touch to change your Apple ID, stay signed in with your Apple ID on only that device.