It's too late to file an extension for 2020. Extension had to be filed before April 15, 2021and only gave you until Oct. 15, 2021 to file. And it is now too late to file to get a refund. But you still have to file and pay any tax due. If you do have a tax due the interest has been adding up so file as soon as possible.
To do a prior year return you have to buy the Desktop program here,
TurboTax 2020 Prior Year Tax Prep - File Past Years' Taxes
You will need a full Windows or Mac to install it on.
If you have a simple return and want to file for free you can fill out the forms by hand. Here are some basic forms.....
Here is the 2020 1040 return
https://www.irs.gov/pub/irs-prior/f1040--2020.pdf
or if you want bigger type use 1040SR for Seniors,
https://www.irs.gov/pub/irs-prior/f1040s--2020.pdf
2020 1040 Instructions
https://www.irs.gov/pub/irs-prior/i1040gi--2020.pdf
2020 EIC and Tax Tables
https://www.irs.gov/pub/irs-prior/i1040tt--2020.pdf
Schedules 1-3
https://www.irs.gov/pub/irs-prior/f1040s1--2020.pdf
https://www.irs.gov/pub/irs-prior/f1040s2--2020.pdf
https://www.irs.gov/pub/irs-prior/f1040s3--2020.pdf
Don’t forget your state. And you will have to print and mail your returns. So be sure to attach copies of your W2s and any 1099s that have withholding on them. You have to mail federal and state in separate envelopes because they go to different places. Get a tracking number from the post office when you mail them for proof of filing.
