You can change through My au (Web).
Please see below for detailed instructions.
Tap on "Smartphone/Mobile Phone" [スマートフォン・携帯電話].
Tap on "Contract Details/Procedures" [ご契約内容/手続き].
Tap on "Contract Information" [ご契約情報].
Tap on "Change" [変更] next to the "Plan" [料金プラン] section.
Select your desired "data plan" [料金プラン選択] and "calling options" [通話オプション] from the "Select Data Plan" [料金プラン] screen.
Tap on "Agree and Proceed" [同意して次に進む].
Check the important notes and tap on "Agree and Proceed" [同意して次に進む].
Review the "Important Notes" [ご注意] regarding the plan change, and put a check in the box for "I have reviewed the above notes" [上記注意事項を確認しました].
Tap on "Next" [次へ].
Choose the method to review the contract details.
Tap on "Apply for Plan change" [プラン変更を申し込む].
The procedure for changing the plan is now complete.
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