The Internal Revenue Service allows taxpayers to adjust the amount of tax withheld from their pay at any time during the tax year. Publication 919 explains to a taxpayer when, why and how to adjust withholdings. According to the IRS, a taxpayer should adjust her withholding if she received a big refund for the previous tax year, or if she owed the IRS additional taxes when her return was filed. A taxpayer should adjust her withholding as soon as she realizes the change needs to be made, to avoid overpaying or underpaying taxes.
Obtain a W-4 form from your employer, or by visiting the IRS Web page and clicking on the W-4 form under Forms and Publications.
Fill out the W-4 form completely to change the amount of your withholding. You can use the worksheet included with the W-4 form or you can use the withholding calculator on the IRS Web page, if you have questions about how many deductions you should take.
Turn the completed form into your company's payroll administrator.
Using the Online Calculator
Go to the IRS Web page; click "Withholding Calculator" under the Online Services heading on the left of the page.
Gather your most recent pay stub and last year's tax return, and click "Continue to the Withholding Calculator."
Select your filing status, choose whether someone else could claim you as a dependent, and click "Continue."
Answer all the questions under the General Information and Credit sections, click "Continue." Answer all of the Income and Withholding questions; use your pay stub and last year's tax return to assist you in completing the information, and click "Continue."
Answer the Deduction questions; click "Continue." This will bring you to a summary page letting you know how many deductions you should claim on your W-4 form.
It is important to have the correct amount withheld, to avoid having to pay interest and penalties if you are not paying enough, and to avoid having too much taken from your paycheck, which reduces your take-home pay.