I am building a method that will build an XLS file and uploading it on user's computer.
I am using this guide:
So code that will define my destination address is:
The code that you have there will save the file on the web server itself, not on the user's computer. You'll need to stream the file down to the user via the browser, and then they will be able to choose where to save it.
You could save the file on the server and then stream it to the user using Response.WriteFile, or you could stream it from memory if you don't want to keep a copy of the file on the server.