If you want to upload your site files to GreenGeeks before your DNS has changed over from a previous host, you can connect via FTP to your server IP address. The IP address is in your account welcome email.
The temporary URL to view your site before DNS changes is the server IP address followed by /~ and your account username. So it looks like this: http://188.8.131.52/~ggexample/
As an alternative to using an FTP program to upload files, you can use the cPanel File Manager.