The IRS limits the time you are able to amend a return to claim a refund to 3 years from the original filing deadline or 2 years of paying the tax due for that filing year if that date is later.
These help articles should help you complete the amending a return process.
How to File an Amended Tax Return with the IRS
Amending Your Income Tax Return
How do I amend my federal tax return for a prior year?
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