You can do the procedure from the au ID Member Information Site「au ID会員情報サイト」.
If you are using a smartphone, tap on the [MENU] icon in the upper 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].
Enter your new password and tap on "Set"[設定].
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