In The Legend of Zelda spin-off Cadence of Hyrule players can either choose to play as Link or Zelda. At the start of the game you’ll have a choice of whether you want to choose one character or the other. But if you want to change between Zelda and Link after that you can, but you have to complete a quest. Regardless of which character you choose at the start, you will need to wake the other one from a deep sleep. This is done with a Hibiscus Potion that you get from a dungeon in Kakariko Village.
The overworld of Cadence of Hyrule is completely randomized at the start of your game we can’t tell you exactly where to go to get to Kakariko Village, but you should mindful of it when you do find it as it will have Zelda or Link sleeping in one of the houses and a graveyard dungeon that you need to visit if you want to get this potion to wake your partner. As you can see in the image below, the big tombstone in the Kakariko Graveyard can be moved to reveal a set of stairs that leads below.
This staircase will take you to a dungeon where you’ll need to complete a boss fight with the Dark version of Zelda or Link. If you want to wake up link you will fight Dark Link, if you are trying to wake Zelda you will fight Dark Zelda. The battle will likely be one of the tougher ones you faced as this boss has a lot of HP and a unique jewel attack. Regardless, if you want to switch characters in Cadence of Hyrule this is the way.
With the potion in-hand head back to the house where Zelda or Link is sleeping and then administer the potion. This will immediately wake the character and unlock them for use. At this point you can change characters at any time by visiting a Sheikah Stone. There is a Sheikah Stone in Kakariko Village if you want to try out the new character. Changing characters mid-game will not take away any of the health items, charms, that you have found before getting to this point. However, some weapon and defensive items will not transfer from character to character.