Instructions
Press "Start" while on the map screen to bring up the in-game options menu, then select the "Deck Build" option. This will show you your available decks, as well as the cards you have stored in your chest.
Move the cursor to the right side of the screen, then press "Start" until you have the deck you want to change. Move the cursor back to the left side of the screen, which is where your Chest cards are.
Scroll through the cards until you reach the one you want to make your deck leader. If the card you want to promote to deck leader is in the deck, highlight it, then press "Square" to remove it.
Highlight the card you want to promote to deck leader, then press "Start." Follow the onscreen prompts to confirm your decision and replace your current deck leader with the new card.