How to change extension .aspx to html?

This may not be such a straightforward thing.  From your post it seems like you want the page in the browser to read html when you are actually serving up an aspx page. 

Normally this is accomplished in one of two ways.  First, you can use url rewriting within your application to rewrite the path to your aspx page.  That way, when the user type the html url, your application intercepts that and serves the aspx page instead.  Here is an article on that:

http://www.stardeveloper.com/articles/display.html?article=2004022801&page=1

remember to go to IIS and re-map the html extension to the .NET dll so that html files get handled by your application.

Second, you could write your rewrite code in an ISAPI filter, that gets called by IIS before .NET does.  The advantage of this is you can change the extension of the request here, and your .NET code doesn't have to do anything, just handle the request.  Here is an example of that....

http://www.codeproject.com/isapi/isapiredirector.asp

I have to let you know, there are quirks and issues with both implementations, so test well.

John Ginzo
GinzoTech Solutions
http://www.GinzoTech.com

i dont want to chang the page type.
i want show a aspx to html.
just in browser(IE,Firefox)

Check out the System.IO.Path.ChangeExtension() method:

http://msdn2.microsoft.com/en-us/library/system.io.path.changeextension.aspx