Your question is not clear. Do you need to complete a 2016 tax return? Or do you need to download a copy of a 2016 tax return you completed using the TurboTax online editions?
If you need to complete a 2016 tax return using TurboTax then you will need to purchase, download and install on a personal computer one of the 2016 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2016 tax return can only be printed and mailed, it cannot be e-filed.
To access your current or prior year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on the Year and Click on Download/print return (PDF)
In TurboTax Free Edition, you can purchase PLUS to unlock your prior-year .tax file.
The exact instructions will vary depending on the status of your 2018 TurboTax Online return (select one):