Go to the Cards menu. Your currently equipped deck will be there ready to edit. If you want to edit a different deck, tap on Change Deck. You’ll see all your decks there. Tap on the deck you want to edit to equip it. Then tap on the Edit Deck button. You can also press to duplicate a deck and edit the copy instead.
Drag cards in an out your deck to edit it. You may change Warlords as well, if you have more than one from the same Legion.
Tap on any card to get more info.
Tap on your deck name to rename it.
Once you have 30 cards, your deck will be automatically saved and equipped. You can jump right away to the Battle screen!