When you're looking at your list of decks, you'll see a little pen icon. If you click on that, it'll switch things up so you can rearrange your decks.

Once you're in rearrange mode, each deck will have a special block on the left side. That block lets you pick up and move the deck around wherever you want it.

Once finished, just click the blue Checkmark icon again and your changes will be saved.

Note!

If there's a deck that's currently being uploaded or processed, you won't be able to click on the "re-order" icon. Basically, you can't rearrange things while something's still being worked on.