Hello,
Amendments can generally be done at any time for any tax year. However, as a general rule of thumb, e-filing amendments can go as far back as 3 years. Anything further back than that in the past will need to be paper filed.
IRS e-file system supports current tax year and the 2 prior tax years of e-filing. Paper filing can be done for amendments further into the past. Keep in mind though that anything further than 3 years will not be eligible for a refund.
For more information, please see:
https://www.irs.gov/e-file-providers/benefits-of-1040-modernized-e-file
https://www.irs.gov/businesses/small-businesses-self-employed/filing-past-due-tax-returns
Best regards!
