You'll have to save your workbook as a memorystream and write it to the HttpResponse. Something like this:
using (XLWorkbook wb = new XLWorkbook())
{
// create your workbook here "MyWorkBook"
// then send it to the http response...
Response.Clear();
Response.Buffer = true;
Response.Charset = "";
Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
Response.AddHeader("content-disposition", "attachment;filename=Report.xlsx");
using (MemoryStream ms = new MemoryStream())
{
MyWorkBook.SaveAs(ms);
ms.WriteTo(Response.OutputStream);
Response.Flush();
Response.End();
}
}