Go to the Collection Menu. Your decks will be displayed there. To edit a deck, press on it and then press on "Edit Deck" on the bottom right.

To the right you’ll have a list of all your owned cards available for that Warlord. Press on any card to see more info about it.

Drag cards in and out your deck to edit it.  You may change Warlords as well, if you have more than one from the same Army. Make sure that after all edits, the deck still has exactly 30 cards. 

On the top-left of the screen you will find the  Filters button. When pressed, it will display the filters panel over the decklist. You can filter cards by Energy cost, Rarity, or card type (Warlord, Troop, Stratagem).