The IRS encourages everyone to use the Withholding Calculator to perform a "payroll tax checkup." The calculator helps you identify your federal tax withholding to make sure you have the right amount of federal tax withheld from your paycheck.
The calculator will ask you to estimate values of your income and other items that will affect your annual taxes. This process will take a few minutes.
If changes need to be made, access a W4 form here: W-4 Employee's Withholding Certificate. Steps one and five MUST be completed for the form to be valid. Changes can be made at any time during the year.