Changing ASP.Net Version for your website, login to Plesk Control Panel under the Home click on the Setup in Hosting Menu.
Now in the Hosting Setup go to Services section select the required .NET framework version and click OK. It will change the default .NET version for your entire site with all your virtual application (configured with virtual directories).
In order to provide running ASP.NET applications which use different .NET framework versions within one domain, Plesk allows setting up the framework version per virtual (web) directory, where the applications are deployed.
To changing .NET Framework Version for a Virtual (Web) Directory click Web Directories in on your Home page.
Select the virtual directory that you want to change the ASP.Net Version.
Enter the required web directory and click ASP.NET in the Tools group.
Now click on Change Version
Then Select the required .NET framework version and Click OK.
Don't Forget to 
from Plesk CP when you are done.
