you would need to recreate the original return item for item line for line so it exactly matches. then save it reopen it and click the amened link. Don't know if you can e-file it.
the general advice is to use the preparer that did the original. if it was their error, it should be done for free. if you used another online self-prepare app it would be easier to use it unless the app doesn't provide for amended returns.
