How do I fix a mistake on a return I already filed?

You file an amended return using Form 1040-X to correct errors or claim something you missed, like a forgotten deduction, credit, or a corrected W-2 or 1099. You generally have three years from the original filing date (or two years from when you paid the tax, if later) to amend and claim a refund. Don't amend for simple math errors — the IRS corrects those automatically — but do amend if your income, filing status, deductions, or credits were wrong. The 1040-X shows the original figures, the changes, and the corrected numbers. Amended returns can now often be e-filed, though they still take weeks to months to process. If the change means you owe more, pay promptly to limit interest. For a small, clear fix, tax software handles it; for a complex correction, a CPA is worth it.