If you feel that you missed a credit, deduction or made a mistake on your tax return, all you have to do it file a 1040X. It's fairly simple to file and the IRS just wants to know what you originally filed for, what the change is and why you are making the change.
Here is the 1040X form:
http://www.irs.gov/pub/irs-pdf/f1040x.pdf
Here are the instructions:
http://www.irs.gov/pub/irs-pdf/i1040x.pdf
Good luck!

