Incorrect billing as per agreement

I would like Autopage to rectify their mistake of more billing as per our agreement.I took a contract of sumsang gallaxy A3 and we agreed on terms and conditions of the amount i have to pay monthly for 24 months.i would like them to correct their billing en bill me correctly as per our agreement.if them fail can they provide me wuth recordings and cancel my contract and collect their device and simcard.Because now the billing is more that wuth what we agreed upon i even asked repeatedly from the sales consultant and she gave me the amount we agreed upon but now is more than that.