Changing the Apple Cash transfer bank account

See the following instructions from Transfer money in Apple Cash to your bank account or debit card - Apple Support

How to update your bank account information

1. On iPhone: Open the Wallet app, tap your Apple Cash card, tap the More button, then tap Card Details.

• • On iPad: Open the Settings app, tap Wallet & Apple Pay, then tap your Apple Cash card. 

1. Scroll down and tap Bank Account, then tap the bank account you want to update.

• • To edit your bank information, tap your routing or account number, add the information, confirm the numbers, and tap Next.5
  • To delete your banking information, tap Delete Bank Account Information. To confirm, tap Delete again. After deleting, you can add your bank information again or add different information.

When you update your information on one device, it automatically updates on all devices that are signed in to your Apple Account.