How can I use the new version of form builder?

I can't find the button for converting to the new form builder.

You can enable it in your account settings page. Here's a screenshot:

You can also create a new form and go back to the My Forms page. After that, select the form and click the More drop down menu then select the Edit in V4.

Hope that helps. Let us know if you need anything else. We'll be happy to assist you.