I simply want to change the Expiry Date on my Account ... How do I do that?

If you're talking about the day a subscription renews, you would have to cancel the subscription, wait for it to end (expire), then start it again on the desired date. Note that for some subscriptions this may entail loss of data (such as playlists for Apple Music) or other features.

View, change, or cancel your subscriptions - Click on the link to learn "How to cancel a subscription from Apple." --> If you want to cancel a subscription from Apple - Apple Support

See -> Related Article: If you need to change or update your Apple ID payment method

Or see -> Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

• If you want to change your card, you need to add first the new card, then remove the old card.

If you update your payment method or add funds to your Apple Account balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.