i want to convert my web application vs 2003 to vs 2010. Can anyone explain step by step.

Life will not be easy though using any tool. You will have to do a lot of changes manually. I tried it once using the default VS2010 functionality and the whole HTML value were missing the quotes for e.g the runat tag was displayed as runat=server instead of runat="server" (don't know if it was problem with my VS or a problem in general). Somehow this didn't cause any issue in compilation. May be after conversion VS knows how to handle this as well. But CSS was breaking at lot of places.

When you try to open a VS2003 code base in in VS2010 it will automatically ask you if you want to change the target framework? (I guess by your question that's what you meant) Click yes and once the conversion is done a log file is created. Do remember to check that out to see what all components or code files couldn't be converted. And the most important thing. Take backup before you start anything. :)