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

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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