If you have a physical, static XML file, then you need to upload it to the www root of your site using FTP. However, this file will be static and will not reflect any changes made to the content tree. I would rather recommend reading and following the documentation you linked. In that documentation you will learn what web part to place on some "sitemap" dedicated page and the web part will ensure that the Google Sitemap XML will be generated dynamically, on the fly.