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

Question

i have created one asp.net website in visual studio 2008 in framework 3.5.when i tried to open this website in visual studio 2010.it reflects the error in web.config file.please do let me know how can i open this website or any other way to change .net framework during the opening of website.

v studio 2008 - defualt framework 3.5
v studio 2010 - defualt framework 4.0
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Manish Dwivedi · Answer

Hi 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

Hardik Patel · Answer

but there is a one problem. i have already opened website with no option as a result in a web.config file some code was generated automatically.like target.netframework = 4. so please tell me how can i solve it?