It won’t let me change my bank details so I can’t update anything

What kind of error message are you getting when you try to update your payment details?

• Change or remove your Apple ID payment information - Apple Support
• If your payment method is declined, or you have billing issue with a previous purchase in the iTunes Store - Apple Support

Hi

Go to iTunes main page bottom of page apple I'd click that view I'd Go to manage payments check / change your credit card details & the 3 digit code from the back of the card.

Cheers

Brian 🇬-1F1E7;