If you posted a Bank Payment with the incorrect VAT/Tax codes.
You must post a bank receipt to reverse the bank payment. Both the Bank Payment and Bank Receipt will show in the Bank Reconciliation window and you must reconcile both transactions which has a net effect of 0. To post a bank receipt to reverse the bank payment:
Bank > Bank Receipts > ensure the following are the same as the original bank payment
You will then be able to enter a bank transaction again with the correct VAT codes.
Please test this in your Salesforce Sandbox to ensure you achieve the desired outcome before you apply the transaction to your live data.