First, be sure that you are actively editing your tax return. If its giving you the option to review or edit the main information dropdowns, click to review any one of them. After you do that click on the menu in the top left corner. Click on the last option "Tax tools". Finally click on "Clear & Start Over" and then the yes button.
As long as you haven't submitted payment, deducted the TurboTax fee from your refund, or registered, you can erase your return and start from scratch in TurboTax Online. Please see the TurboTax FAQ below for instructions: