Just tell them you wont be re-newing your contract with them and it will end on the given date. Make sure you cancel the direct debit with your bank though as they have a habit of just carrying it on and still taking payment out, even though you cancelled it or let it lapse, orange are very guilty of this, which is one of the reasons why i will never go back with them no matter what deal they offer me!!!

Ive found you can go over by a few weeks and they just charge you pro rata.