Where can I upload a previous tax return that was not filed through turbo tax

If you used tax preparation software to complete the tax return you should be able to get a copy of the return.  Or if you used a paid tax preparer they can give you a copy.

 

Otherwise -

 

Go to this IRS website for free federal tax return transcripts - https://www.irs.gov/individuals/get-transcript

 

For a fee of $43 you can get a complete federal tax return from the IRS by completing Form 4506 - http://www.irs.gov/pub/irs-pdf/f4506.pdf

What exactly do you mean by "upload" a previous tax return?  What are you trying to do?   Are you trying to put 2019 data into TurboTax  to start a 2020 return?   Or do you mean you just need a copy of a previous tax return that you prepared somewhere else?