The employee's Form W-4 details need to be entered in each employee's Federal Taxes section in order to calculate federal withholding accurately.
To edit an employee's W-4 details:
Go to Employees
Click on the employee's name
Go to their Federal Taxes tab
Edit then save changes
The next payroll run will use the new values to calculate their federal withholding.
Employees need to fill out a W-4 form when they are hired or if they choose to make updates.