If you need to change your withholding amount then you will need to make those changes to the IRS Form W-4, Employee's Withholding Allowance Certificate that the IRS has on file for you. However, if you need to make changes to the number of dependents that you claim you will need to complete a new W-4. You are required to fill out a new W-4 every time you start a new job, and you can request to change the information on your W-4 form if you need to. Just tell your employer that you need to update your W-4 and they will probably give you a new one to fill out.