Last year I used Turbotax last year mistakenly. I am non-resident. Am I eligible to use the TurboTax amend software to amend my last year's filing?

[nauhoque]

This is my 4th year in the US.

[LinaJ2020]

Yes, you can amend your 2018 taxes with TurboTax. 

Here are the steps:

You would need to submit both the Form 1040-X and Form 1040NR together:

1. Download the Form 1040-X from the IRS website (see below).  On the front page of Form 1040-X, enter your name, current address, and social security number (SSN) or IRS individual taxpayer identification number (ITIN)
2. Do not enter any other information on page 1.  
3. Do not complete Parts I or II on page 2 of Form 1040X.
4. Enter in Part III the reason why you are filing Form 1040X.
5. Complete a new or corrected Form 1040-NR.  As TurboTax does not support nonresident forms, click here Sprintax.
6. Across the top of Form 1040-NR, write “Amended”
7. Attach the new or corrected return (Form 1040NR) to the back of Form 1040-X. 

https://www.irs.gov/pub/irs-pdf/f1040x.pdf 

https://www.irs.gov/pub/irs-pdf/i1040x.pdf, page 7 under Residents and Nonresidents

@nauhoque

[nauhoque]

Thanks for your reply. I filed both of my tax filings (federal and state) through TurboTax.

Do I have to submit two 1040X forms (1 for federal, 1 for state)? 

[KarenJ2]

For your state tax return, you will need to go to your state website and download a copy of the amended return for that state.  You will need to prepare a nonresident state return in Sprintaxand have it printed so you can mail the correct return with your state amended return. 

Sprintax may be able to assist you in filing a state amended return.