How can I fix a mistake I made on my income tax filing?
Depending on your mistake, you probably need to file an amended return (called a "1040X" form).
The above pages include links to the 1040X PDF form, and the instructions for it.
If you filed with TurboTax, it supports the ability to do a 1040X (not sure about TaxCut, etc).
Note that you have to print/mail the 1040X form, you can't e-file it.