This article will be useful if you want to learn how to change / replace the deck, see what decks you already have, return the old deck if you chose the wrong one.
First you need to go to the game settings.
Next, go to the "Decks" tab and choose the deck that you like the most. Please note that the row with decks can be scrolled through.
New card covers can be obtained from the first month of daily rewards, by completing the collection, or purchased through special sales.