would like to update my credit card details

You can change card details yourself, you don't have to contact iTunes Support (and these are user forums, not iTunes Support) : Change or remove your Apple ID payment information in the iTunes Store - Apple Support

If you want to contact iTunes Support : http://www.apple.com/support/itunes/ww

If you read my above reply you will find a link for contacting iTunes Support. But if your cards are not valid they are unlikely to be able to help.