Convert from 3.5 to 4.0 framework

What is the most trouble-free way to convert a asp.net webSITE from framework 3.5 to 4.0?

I tried opening in VS 2010 and answer yes to converting to framework 4 but I had major problems with many webserver controls. One example is the ListView as VS 2010 would not recognize the ListView and gave me errors like 'ListView is not declared....

Thanks.