Double billing

Good Day <br> <br> i had to pay a double billing where i have paid in December when i upgraded. they did not tell me that i still have to pay 2 months for the tablet that i have upgraded i was only told that they will take off in December for the rest that i owe them. So i dont know why they tell you that you can upgrade like a month or 2 before the time if they still charge you for the two months? The way i feel now i want to cancel my contracts.