You'll need to amend your previous years' returns to a 1040NR.
Please read this TurboTax answer which addresses your question.
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Thank you for your help on this matter. I found now two separate threads about amending the return.
1. describes steps with using sprintax (https://ttlc.intuit.com/community/tax-credits-deductions/discussion/international-student-filed-with...)
2. describes steps filling it and mail it (https://ttlc.intuit.com/community/after-you-file/discussion/i-am-an-international-student-and-mistak...)
Which of them is now correct? Can I use sprintax to file my amending?