If you are using the CD/Desktop version of Turbotax which you install and run on your personal computer, just click on "File" at the top left of your screen, then click "New".
If you are using the Turbotax Online version which you access through your web browser, you can prepare only one return per account/payment. This Turbotax help topic tells you how to start another return:
https://ttlc.intuit.com/questions/1894512-how-do-i-start-a-second-return-in-turbotax-onlineThat is a fundamental difference between the Turbotax Online version, which you access through a web browser, and the CD/Desktop version which installs and runs on your home computer.
With the CD/Desktop version, you can prepare as many returns as you wish just for the price of the box.