From the left side menu, go to Team > My Team > Employee Information and choose the employee you would like to update.

From the employee profile, go to the Payroll Tab > Tax Information.

From tax information, choose which tax jurisdiction you would like to update (i.e. federal or state). 

After tax jurisdiction is selected, go to +Add. 

A pop up window will appear and allow you to make the necessary updates based on the federal or state forms the employee has submitted to you. Be sure to update the Date From section. Once updates are complete, select Add to save changes.