How do I save information in the online version as a .tax file and then upload it to my desktop version?

How far are you?  Have you already imported the csv file into an online return?   If not I have more steps we can do first.   

You do not import the .tax2022 file from online.  You just download it and OPEN it in the desktop program.  So if you already  started a desktop return you can't import to it.  The online return is now the tax file.   

First save your return as a tax data file from Online
https://ttlc.intuit.com/community/saving/help/how-do-i-save-my-turbotax-online-return-as-a-tax-data-file/00/25740

See how to switch to the Desktop program,
https://ttlc.intuit.com/community/choosing-a-product/help/how-do-i-switch-from-turbotax-online-to-the-turbotax-software/00/26129

After you get the program reinstalled the first thing to do before you open your tax return is to update the program and install any state programs you had. Then open your file. So you first might need to start a fake return to be able to download the state program (go to FILE - NEW). Enter some basic information, then go to the state tab and download the state program. Then try to open your existing return, go to FILE - OPEN