Instructions
Load "Duels of the Planeswalkers" to the main menu. Notice that in the bottom right of the screen there is a deck name for the deck you currently have loaded. Next to the deck name is an image of what colors make up that deck.
Click on the deck, this will open the deck manager window. Here you can scroll through which deck you'd like to edit. Once you've selected the deck you'd like to edit, choose "Edit deck" in the bottom right corner.
Drag unlocked cards from the deck to the sideboard to take them out of your deck. Once you've adjusted what card you want in your deck, make sure to save. The game automatically adjusts the number of basic lands in your deck to accommodate its final card count.