Did you forget to take all the deductions you could have, or did you make some mistakes in preparing your tax return? You should file an amended tax return
if you need to correct your filing status, total income, tax deductions, or tax credits. You should not file an amended return if you are only correcting math errors, as the IRS computers will fix math errors automatically. If you forgot to take any deductions or credits, you have three years from the original filing deadline to request an additional tax refund.