The instructions to amend Form 1040-NR are the same as Form 1040 or 1040-SR.
Per the IRS Instructions for Form 1040 and 1040-SR, page 80:
File Form 1040-X to change a return you already filed. Generally, Form 1040-X must be filed within 3 years after the date the original return was filed or within 2 years after the date the tax was paid, whichever is later. But you may have more time to file Form 1040-X if you live in a federally declared disaster area or you are physically or mentally unable to manage your financial affairs. See Pub. 556 for details.
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