In order to edit your deck and the cards it contains, tap the menu icon (upper left) and then tap Planeswalkers. Select your Planeswalker and then tap Custom 1. This will show your current cards free deck.
From here you choose "Edit" to switch out cards you have by tapping the green check mark of the cards in the first tab called Deck. Select “Card Pool” to go through the cards you own and select one to place into the deck.
You can select the Blue button with the x over the cards to remove all the cards you have in your current deck. Red X button will cancel the edit, and a confirmation will appear to make sure you are ok with not making any changes. The green button means you are done editing and want to save your deck. The game may tell you the deck isn’t ready if you don’t have enough cards in it or you are over the limit of how many creatures, spells and/or supports you can have per your level.
Note that the cards here will only be the cards that match or work with the Planeswalker’s color. Also notice that the top left shows how many creatures, supports and spells your Planeswalker can have for the deck, this number changes as the Planeswalker levels up.
Please note, players can choose "Quick Improvement" at anytime to improve their deck if they so choose.