CREDIT NOTE FOR SALES TAX/VAT

WE WANT TO PREPARE CREDIT NOTE FOR VAT WRONGLY CHARGES IN THE BILL. WHEN WE ARE PREPAIRING THE DOCUMENT (INVOICE CORRECTION) WITH REFERENCE TO THAT BILLING DOCUMENT, THE DIFFERENCE VALUE OF(TAX/VAT) IS NOT SHOWING IN NET VALUE AND IT IS NOT PROCESSING FOR INVOICE(CREDIT NOTE).SO HOW TO PREPAIR CREDIT NOTE FOR WRONGLY CHARGED VAT?

Please guide .