You can change it from the au ID Membership Information Site [au ID会員情報サイト].
Please see below for detailed instructions.
Tap on the [MENU] icon in the upper right corner of the screen, and tap on "au ID membership information" [au ID会員情報].
Tap on "Change" [変更] for the contact email address.
Tap on "Change" [変更] or "Register" [登録] for email address 1 or email address 2.
Enter the email address you want to change or add and tap on "Send confirmation email" [確認メールを送信する].
An email will be sent to the email address you entered. Enter the verification code in the email and tap on "Settings" [設定].
The process of changing the contact email address for your au ID is now complete.
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