1. Install the Preview 2 on RTM VS 2012
2. Open your existing LS application
3. Right click on your Project and you will see an option called "Add Client"
4. Click on that to add the NEW HTML Client
5. NOTE: Your project will be upgraded
Check out the link
http://msdn.microsoft.com/en-US/vstudio/htmlclient to see the HTML Client Developmental experience
Sandeep Kondam, SDET Visual Studio LightSwitch Team