F-1 Student Taxes Filing

Hi,

I came to the US as a F1 student in 2016. So I was a exempt individual for the first 5 calendar years (2016-2020).

Now it is 2021 and my 6th year, so I am considered a resident for tax purposes this year in 2021. But I want to file for 2020 tax return. Do I file as a resident (1040) or a non-resident (1040NR)? 

If I have to file 1040NR, can I use the First-Year Choice to file as a resident for 2020? 

Thank you!