How do I remove App Store balance to add a bank card?

I am not understanding the problem. You can have a payment card and account balance. To see the payment methods that you can use with your Apple Account [formerly Apple ID], click here --> Payment methods that you can use with your Apple Account - Apple Support

In general the only way to get rid of Apple Account balance is to either spend it or to ask Apple to keep it. ➔ "You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards, or unused Apple Account balance for cash, except as required by law. If your jurisdiction allows for refund of redeemed gift balances, you can contact Apple Support."

From: Check your Apple Account balance - Apple Support and Change your Apple Account country or region - Apple Support

As I said though, there is no need to do either of those if all you want to do is add a bank card to your account.

If you are having problems adding a card then we need a better description of the problem such as what exactly is happening and the exact wording of any error messages you see.

Read this article about how to view your current payment types and add Apple Account payment methods on an Apple mobile device, a Mac, or a PC --> Add a payment method to your Apple Account - Apple Support

If an item appears in red then it is no longer working and you either need to replace it or get it working again. Even if an app is free, or payment will ultimately be taken from your personal account balance, Apple may absolutely require the presence of a valid and working payment method such as a credit or debit card.

If you can't add a payment method, check Add a payment method to your Apple Account - Apple Support , or this one about what to do if your payment method is declined in the App Store or iTunes Store —> If your payment method is declined in the App Store or iTunes Store - Apple Support