You can complete the procedure through My au (Web).
Please see below for detailed instructions.
Tap on "Billing and Payment Procedures List" [請求・支払関連のお手続き一覧].
Tap on "Payment of Charges" [料金のお支払い].
Tap on "Check Remaining Installment Amount/Lump-Sum Settlement" [分割支払残額の確認・一括清算].
Check the button for either "Telephone Number with Installment Contract" [割賦契約のある電話番号] or "Contract" [契約] and tap on "Select" [選択].
Tap on "Lump-Sum Settlement of Remaining Installment Amount" [分割支払残額一括清算] in the Important Notes.
Tap the button for the "Installment Contract You Wish to Settle" [清算したい割賦契約] to check it.
Tap on "Agree and Proceed to Next" [同意して次へ進む].
Review the displayed information and tap on "Apply with This Information" [この内容で申し込む].
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