I need to change my credit card number

Set up your Apple ID payment method and shipping address on Mac https://support.apple.com/guide/mac-help/set-apple-payment-method-shipping-address-mac-mchl29cac995/mac

On iPhone: Settings > Name > Payment & Shipping

There are several ways to change you payment method, including on your computer, or iOS device, or also through the web browser. Attached here is a linked support article: https://support.apple.com/en-us/HT201266

If you want to change your payment type on an iMac, you still need to go in through the App Store app, as outlined in the support article instructions.