You can complete the change procedure from the "au ID Member Information Site" [au ID会員情報サイト].
On the smartphone, tap on [MENU] icon at the top right corner of the screen.
Tap on "au ID Member Information" [au ID会員情報].
Tap on "Change" [変更] on the right side of the "Password" [パスワード] section.
Enter your new password and tap on "Set" [設定].
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