Navigate to My Info > My Pay > Forms > Withholding

Once on the withholding screen, if you want to update your existing withholding, click on Add New in the right hand corner and then select +Add next to the appropriate form that you want to fill out.  

After you complete all applicable fields, click on Review in the top right.

The form will then be grayed out.  Review the form for accuracy. If you find errors, click on Edit and make the appropriate Edits.  If it is accurate, Sign & Submit.

A box will pop up asking you to enter your password for submission.  Enter your password and then select I Agree to submit the form to your company admin.