bool AccessDeniedflag = SPSecurity.CatchAccessDeniedException;
SPSecurity.CatchAccessDeniedException = false;
try
{
SPList list = web.GetList(web.ServerRelativeUrl+"/Lists/ListName");
SPListItem item = list.Items[0];
item["title"] = "ayman-elhattab.blogspot.com";
item.Update();
}
catch(UnauthorizedAccessException ex)
{
// Redirect to your custom page
SPUtility.Redirect("MyAccessDeniedPage.aspx",
    SPRedirectFlags.RelativeToLayoutsPage,this.Context);
}
finally
{
SPSecurity.CatchAccessDeniedException = AccessDeniedflag;
}