What BP-compatible theme are you using? If you’re using the bp-default theme, you should not make changes directly on that theme because it will be overwritten when you upgrade BP plugin. You need to create a child theme instead.
Copying footer.php from bp-default theme into child theme folder per tutorial (if you want to use bp-default theme and customize it) and upload to server is just like copying over some a file from one folder to another in your computer then FTP/cpanel upload to server.