File download with save as dialog box from web browser in VB

Nirav, if I understand correctly, you want to send the .XLSX file from the server to the browser, but to give the user the option to SAVE the file rather than have it automatically open in Excel?
If that's the case, then in your server-side VB.Net code, do something like:

Dim filename as string = "demo.xlsx"                    '' 
        Dim fullfilename as string = server.mappath([filename]) '' Get the full local filesystem filename
        Response.Clear()                                        '' We don't want to send any of the current page back
        Response.Headers.Add("Filename", filename)             '' Set the filename 
        HttpContext.Current.Response.AddHeader("Content-Disposition", String.Format("attachment;filename={0}.xlsx", filename))  '' This tells the browser that the content is an ATTACHMENT, i.e. to save it, not open it
        Response.ContentType = "vnd.openxmlformats-officedocument.spreadsheetml.sheet"  '' The Content-type for XLSX spreadsheets
        Response.WriteFile(fullfilename)                         '' Send the actual file content
        Response.End()                                           '' End the response - nothing else gets sent

The important part here is the content-disposition, which tells the browser to "download" rather than open it. How the browser deals with this is browser-specific; Chrome will always put it in the Downloads folder, IE may prompt you for a location. Hopefully the above will help you, even if only with some keywords to research further.

Good luck!