Amending a Withholding Return
If you overpaid for a period, you may take a credit on a future return in the current calendar year. The credit may be taken on a return/payment filed through Revenue Online or a subsequent paper form
If you are unable to claim the credit on a subsequent e-file or paper return within the calendar year, you should claim a refund on your Annual Transmittal for the appropriate year
If you owe additional tax, file another return for the period the tax is due reporting only the additional amount owed through Revenue Online. If you cannot file electronically, file the appropriate paper return reporting only the additional tax due for the period.