Employees can make changes to their W-4 at any time. Changes are made in Paycor. Login to Paycor and Click on profile picture (top right) > Pay Stubs & Tax Docs > Taxes > FITWH (Federal Income Tax) or NE (Nebraska state tax) > Update Withholding > Fill in the changes that you want to make > Save.
If you are wanting an additional amount withheld, click on Adjust Withholding > Add to Withholdings.