You cannot change a tax year to a prior year, the Online version is only to file the current year's tax return, which now is 2020.
To file a prior year return with TurboTax, you need to download their 2018 desktop software. The return would need to be mailed in. To purchase the software from TurboTax click on the following link:
https://turbotax.intuit.com/personal-taxes/past-years-products/
