It's too easy, let me help you.
In the main menu, click on "collection". Then click on "Decks", you'll see the decks of your heroes. Simply you can click on the "pencil icon" to edit the deck, or you can create a brand new deck by clicking on "new deck"
To change hero, select a battle (multiplayer battle or solo mission) then you'll see "choose a deck" tab. Click on it and you'll see your decks. Choose one of them and you're good to go!
