Apple Cash doesn’t give option to choose account # or routing number

See the following excerpt from an Apple Support article for information about changing the bank account. From: Transfer money in Apple Cash to your bank account or debit card - Apple Support

How to update your bank account information

1. Go to your card info:

• • 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. Tap Bank Account, then tap the bank account you want to update.

• • To delete your banking information, tap Delete Bank Account Information. Tap again to confirm. After deleting, you can add your bank information again or add different information.
  • To edit your bank information, tap next to your routing number or account number, add your information, then confirm the numbers and tap Next.6

When you update your information on one device, it automatically updates on all of the devices that are signed in with your Apple ID.