You can update an existing file by uploading another with the same name. The old one will be replaced, and you'll keep the download stats too.
Ah! I'm glad that's an option, it's exactly what I needed ^^ Thank you very much!
Do you know about the other part? Whether people can download multiple times the stuff they bought.
William already answered that question, and he's right: Once people have bought a game, they can download the files as many times as they need.
Ah!! It slipped my mind right now <_< Sorry for repeating myself. And thanks again to both!