I've already published some albums. Where do I go to change the download rules?

Open the album's Edit page and find the Downloads section — there's an Edit button next to it. That opens the Download Settings modal where you pick between free downloads, paid downloads, or none, and if you choose paid, you set whether it's per photo or a one-time album price plus the amount. It applies to the album going forward, so you can flip a published album to paid or free at any point without touching the photos themselves.