My apple account wont let me update my card details

Re: Apple Account: I need to update my card

If you have active subscriptions, you may need to add a new card before you can delete an old card,

This is because you must always have a payment method available so subscriptions remain "active".

See: How to change an Apple Account payment method:

If you need to change or update your Apple Account payment method - Apple Support

If needed:

How to add an Apple ID payment method:

Add a payment method to your Apple Account - Apple Support

How to delete an Apple ID payment method:

Remove a payment method from your Apple Account - Apple Support