Per the instructions for 1040NR, US Nonresident Alien Income Tax Return, page 57:
How Do You Amend Your Tax Return?
File Form 1040X to change a return you already filed. Also, use Form 1040X if you filed Form 1040NR and you should have filed Form 1040, or vice versa. Generally, Form 1040X 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 1040X if you live in a federally declared disaster area or you are physically or mentally unable to manage your financial affairs. See Pub. 519 and Pub. 556 for details.