1. When logged in to Seller Central, navigate to the Add a Product page.
In the Inventory menu, select Add a Product2. At the bottom of the page you will see an orange bar with a link to "Try it out". Click the link.

3. You should see a cyan colored bar at the bottom indicating that you're using the new UI.

If you already had the cyan bar, then you're already using the new version and you don't need to do anything else.