If you have had the contract for more than 12 months and have not had an upgrade to it in the last 12 months simply give your provider the usual 1 month`s notice of cancellation and then buy a Pay-as-you-go sim card in place of the contract one, readily available from �8.99 from http://www.accomms.com/ If the phone has been locked by your provider you must either stay with them or have the phone unlocked to accept any other sim card, easily done at most local markets.
If you are on Orange and as long as youve had your number for over 12 months then you can change it to a pay as you go phone for free and keep your same number. But once you have done this you cant change it back to contract.