how to open .net framework 3.5 asp.net website in 4?

When you open a Web project that has not been opened in Visual Studio 2010 before, you are asked if you want to upgrade the site to the .NET Framework version 4. If you click Yes, Visual Studio makes the required changes to the Web.config file.

If you click No, Visual Studio determines the targeted version of the .NET based on what it finds in the Visual Studio project file or in the Web.config file.

for changing the targetframework version of existing application follow this link

http://msdn.microsoft.com/en-us/library/bb398791(v=vs.100).aspx#additional_information