There are two ways of changing the .NET pool when you are using Orchard CMS / razorC.net
1) If you are using the IIS Remote Manager please follow these steps:
-Open IIS Manager. For information about opening IIS Manager, see Open IIS Manager (IIS 7).
-In the Connections panel, expand the server node and click Application Pools.
-On the Application Pools page, select the application pool that contains the application that you want to change.
-In the Actions panel, click View Applications.
-Select the application whose application pool you want to change and then click Change Application Pool in the Actions panel.
-In the Select Application Pool dialog box, select an application pool from the Application pool list and then click OK.
2) If you are using the WebSite Panel: Log in the WebSite Panel -> Click on WebSites -> Click on the particular website name-> Click on tab
"Extensions" and set the ASP version to 4.0