Who changes the dates with out consultation

Please explain where in the contract it says you may change the billing cycle. I have not agreed to this and as such if you take of this new amount, I will be coming through to Midrand and you will not appreciate my manner, just a I do not appreciate your arrogance in this matter. I am sure by now you have loaded your debit order run, so with in the next few days you can expect a visit and I will be leaving with credit,.