You'll need the tax data file of the return you started in TurboTax Online, as well as the TurboTax CD/Download software.  If you've already paid for TurboTax Premier Online, you'll need to speak with our phone support team to review your options:  What is the TurboTax phone number?

Refer to the following Help Article for instructions to download your .tax file:  How do I save my TurboTax Online return as a tax data file?

If you already started in the Online version you can move your return to the Desktop program even without paying for Online.  If you already paid for the Online version you can contact them to give you the program download.  

First download the .tax file from the Online version,  

https://ttlc.intuit.com/community/saving/help/how-do-i-save-my-turbotax-online-return-as-a-tax-data-...

Then see, How to Switch to Desktop,

https://ttlc.intuit.com/community/choosing-a-product/help/how-do-i-switch-from-turbotax-online-to-th...

After you get the program installed the first thing to do before you open your tax return is to update the program and install any state programs you had.  So you first might need to start a fake return to be able to download the state program (go to FILE - New)

Then you just OPEN the .tax2020 file, go up to File-Open.  You don't import or transfer it.