Get your taxes done using TurboTax

What?  Either you misunderstood them or they misunderstood you.  You can not go back and have your employer take out any more from your paychecks for 2015.  2015 is over.    You give the employer a W4 to tell him how much to withhold.  If you fill out a new W4 it will only be for right now for the future.