To delete a return in TurboTax Online 2017 you first need to go to Tools>Return Manager. Then you need to open another return - not the one you want to delete. Then go back to Tools>Return Manager, and click the trash can next to the return you no longer need.
If you don't have any other returns in your account, then you can create a new one by clicking "Continue" at the bottom of the page next to "Start a new 2017 return". Once you get to the "Welcome to TurboTax" page, you can go back to Tools>Return Manager and delete the other return.