I'm having a hard time upgrading my iCloud credit card

See this article about updating payment methods on various platforms: If you need to change or update your Apple ID payment method - Apple Support

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

Update your payment method on your Windows PC or Microsoft Surface

1. Open iTunes.
2. In the menu bar at the top of the iTunes window, choose Account, then choose View My Account.
3. Choose Manage Payments. (If you're using an older version of iTunes, tap or click Edit next to Payment Type.)
4. Choose Add Payment. 
5. Enter your new payment method, then choose Done.