It is not possible to run two different versions of ASP.NET in the same IIS process

Server Application Unavailable

The web application you are attempting to access on this web server is currently unavailable.  Please hit the "Refresh" button in your web browser to retry your request.

Administrator Note: An error message detailing the cause of this specific request failure can be found in the application event log of the web server. Please review this log entry to discover what caused this error to occur.

You will also receive Event ID 1062 in Event Viewer that says:

"It is not possible to run two different versions of ASP.NET in the same IIS process. Please use the IIS Administration Tool to reconfigure your server to run the application in a separate process."

The simple solution for this is:

Create a new application pool and move the site that you will be upgrading to that pool. Once you have placed the site or virtual directory in new application pool, then upgrade to the new framework version.

Follow the below steps to create resolve this issue: