how do i cancel an itunes voucher on my old one and redeem it on my new iphone?

i have recently recieved an iphone 4s. after spending many hours etting it up i have finally finished stting it up. i have been given a voucher for itunes but redeemed it using my old iphone, now i cant redeem it on my new one. how do i cancel it on my old phone and redeem it on my new one?