How can I edit an existing custom spread?

In your custom spread list, find the little pencil icon on your spread.

You'll be brought to the edit spread screen, which will look very similar to the creation screen. Here you can add cards, order them, and delete them. Tap on each card to edit it. You can change position names, and its description, as well as delete the card from the spread. Tap the X button at the top right corner when you're done.

When you've completed your edits, tap the heart icon on the bottom left corner. Make any edits to the spread info here and then confirm. You'll be brought back to the custom spreads page.