There is no way to upload your past-year tax return files from your computer to TurboTax Online. All you can do is transfer information from the immediately preceding year when you start your tax return in TurboTax Online. If you start a 2018 tax return online now, you can transfer the information from the 2017 tax return on your computer (the .tax2017 file). There are instructions in the FAQ at the link below. In the FAQ, click the blue link that says "If you used the TurboTax CD/Download software last year."
Most people who are accustomed to using the CD/Download TurboTax software are not satisfied with TurboTax Online. It lacks many of the features of the CD/Download software. The most significant is that TurboTax Online does not have forms mode. It's also much harder to figure out problems or trace the source of an entry in TurboTax Online.