If you filed your 2002 tax return and have since realized that you made a mistake, fill out Form 1040X -- Amended U.S. Individual Income Tax Return. It's a simplified version of the standard Form 1040 that lets you make the necessary adjustments. It pays to wait a few weeks before sending the amended return -- it's easier if the IRS has already received and begun processing your first return.
Big note: if you generate a balance due with your amended return, interest and penalties will be calculated starting April 15.