Unable to update my card information/payment method on Apple Account

Has a previous charge fail to process on that card? If so, Apple may have considered that card no longer a reliable payment method and you will need to add a different one. You can contact Apple directly here, since we have no access to any of your account information.

Contact Apple Support - Apple Support

It would help to have more details than simply, "it won't let me."

See this article for information about updating payment methods on various platforms (iPhone, Mac, etc.): If you need to change or update your Apple Account payment method - Apple Support

Here is how you update your payment method on an iPhone or iPad:

1. Open the Settings app.

2. Tap your name.

3. Tap Payment & Shipping (formerly Media & Purchases). You might be asked to sign in with your Apple ID.

4. Tap Add Payment Method, enter the new payment method, then tap Done.

5. Tap Edit.

6. Tap the red delete button next to the old payment method, then tap Delete.

Also note:

1) ”If you change your address, the credit or debit card information that you use with your Apple Account might be automatically updated by your financial institution, and you don't need to take action.”

2) “If there's an issue [such as you having a subscription or being a Family Sharing Organizer], add a new card first, then remove the old card."

If an app or feature on your device is not working, not appearing, or is restricted (grayed out), it may have been disabled using Screen Time, or by an IT administrator if you are using a device owned by an employer or educational institution.

- Read this support article If your Apple ID or other options are greyed out in Settings —> If your Apple Account is grayed out in Settings, or you can't sign out - Apple Support

Tap Settings > Screen Time > Content & Privacy > scroll to Allow Changes and turn on.

- If the device is managed (Settings > General > VPN & Device Management), contact the IT department that is doing the managing.