The instructions to amend Form 1040-NR are the same as Form 1040 or 1040-SR.
Per the Instructions for Form 1040 and 1040-SR, on 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.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.