Every duel in Legends of Runeterra starts with the Mulligan phase, which is a card selection stage. Players draw 4 cards and have the option to decide which ones to keep and which ones to replace for other random cards from the deck.
If you want to redraw one of the cards just press the "Replace" button. After selecting the cards you want to exchange, you can also change your decision and click the "Undo" button. The exchange of cards does not result in any penalty, but allows you to get a better Mana curve. The key to success in Legends of Runeterra is the ability to play cards from the very beginning of the duel, so it's worth having units with a low Mana cost in the opening hand.