If you subscribed via our website and are paying ITV for your subscription, head to itv.com and go to the manage account area by clicking your name at the top of the page (you'll need to sign in if you're not already signed in).
Click on change payment details and enter your new details. Once you've saved your new card you'll be sent an email confirming the change.
If you subscribed via the iTunes store on your iPhone or iPad, you'll need to contact Apple to change your payment details. You can visit them here.