First, you need to log in to your app builder and open the app that you want to preview. After logging in, please click on the Design Smartphone button shown on this image:
On the next screen, among other options that allow you to select homescreen layouts, and design your main menu, you can see the live HTML5 app preview. It is visible on this sceeen:
In the HTML5 preview, mouse clicks replace "taps" and click and drags replace "swipes":
Now you're free to test your app as you'd see it in the HTML5 app version. However some features that use device capabilities are not acessible inside the live preview. These are QR code reader, Radio streaming and Camera / picture upload.