I'm a non-resident alien who's working under OPT (F1 Visa) and has been lived in the states less than 5 years. (1040NR Eligibility)
I e-filed a Federal Extension thru Turbotax and received an approval mail.
Is that a successful extension filing?
If it's not, how should I file an extension?
It is a successful filing. The next step would be to amend your return.
Please wait until your return is fully processed and you have received your refund or your payment has cleared. To amend, you would need to submit both the Form 1040-X and form 1040NR together:
https://www.irs.gov/pub/irs-pdf/f1040x.pdf - form
https://www.irs.gov/pub/irs-pdf/i1040x.pdf - instructions
(See page 6 under Residents and Nonresidents)