The mulligan (exchange) phase is the first stage of any game. It is now that the Player has to make the first important strategic decisions. You can exchange up to 6 cards, but if you don't feel the need, you can give up this privilege.
Search for important cards
It's always worth looking for important cards that can contribute to winning a duel. Usually these are golden, special units with powerful skills.
Pay attention to the skills of the units
During the exchange, focus on the skills of the units. For example, the Regiment Drummer is of no use to you if you have all the possible Loyal units in your hand. In this case, send at least one Loyal unit to the deck to take advantage of the Drummer skills.