You can change your plan on My au (Web).
Please see below for detailed instructions.
In the "Plan" [料金プラン] section, tap on "Change" [変更].
Select your desired "Plan" [料金プラン] and "Calling Plan" [通話オプション].
Tap on "Next" [次に進む].
Review the changes and important notes, then tap on "Agree and apply for plan change" [同意してプラン変更を申し込む].
Check the box for "I have read and understood the above important notes" [上記注意事項を確認しました] and then tap on "Next" [次へ].
The plan change is now complete.
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