Unable to update payment method for iCloud subscription

It helps if instead of just saying you "can't" do something you also mention what makes you think that and any error messages you see.

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.

If you are a member of Family Sharing but not the Organizer, read these about why you cannot add a payment method:

How to share apps and purchases with Family Sharing - Apple Support and How apps, content, and subscriptions from Apple are billed - Apple Support