You need to check the terms of your contract. You should have been given one at the time you signed up. It is probably for 12 months and strictly speaking they are within their rights to hold you to that i.e. continue to charge you that rent. You could however try to change it by either going back to the shop you originally purchased from and ask them to try or contact O2 (Easy using their web site.). and ask them if they will allow any modifications. I should warn you that if they agree be very careful that you do not end up signed up for (and paying) on two contracts