I assume you are still in the preparation phase and haven't yet filed.
Your return has to be open before you will see "Tax Tools" in the left menu column.
The link below has a FAQ with a Youtube video.
If you've already paid (or registered in Free Edition), it won't let you clear and start over, but we can tell what you could do instead if you can't. So report back if that happens to you.
https://ttlc.intuit.com/questions/2586254-how-do-i-clear-and-start-over-in-turbotax-online