scroll to the top of the page after user change the page in the gridview

Sys.WebForms.PageRequestManager endRequest Event

Raised after an asynchronous postback is finished and control has been returned to the browser.
Syntax

Sys.WebForms.PageRequestManager.instance.add_endRequest(endRequestHandler)
Sys.WebForms.PageRequestManager.instance.remove_endRequest(endRequestHandler)

endRequestHandler: The name of the handler method that will be called.

The endRequest event is raised after an asynchronous postback is finished and control has been returned to the browser. You can use this event to provide a notification to users or to log errors.

http://www.asp.net/AJAX/Documentation/Live/ClientReference/Sys.WebForms/PageRequestManagerClass/PageRequestManagerEndRequestEvent.aspx