I assume you are preparing a 2018 return using TurboTax Online.
If your return is not already open, then log into your account and choose "Take me to my return."
When the return is open, then at the Welcome screen or at any screen in the open return, go to the left menu column and find TAX TOOLS. (see image below)
Expand that section and choose "Clear and Start Over."
NOTE: If your left menu column only has choices for Tax Home, Previous Taxes, Intuit Account, and Sign Off, then you are at the wrong screen. Your return must be open to get the full left-column menu.
