Game Freak hasn’t made it particularly clear how to change the Pokémon in your party in Pokémon Legends: Arceus. Encouraged to constantly catch and battle every Pokémon that you see in your mission to complete the Hisui region’s first-ever Pokédex, you will regularly find yourself wanting to swap out your Pokémon.
The bad news is that it isn’t as straightforward as you are probably hoping. While you have the ability to access your Pokémon Boxes largely wherever you are in the recent Pokémon Brilliant Diamond and Pokémon Shining Pearl, in Pokémon Legends: Arceus you will have to speak to specific characters to change the Pokémon in your party.
When you are out surveying each area of the Hisui region, you will need to speak to a Galaxy Member. These characters are located at the Base Camps that are established as you continue to progress in the game, and which you will eventually unlock the ability to fast travel to. Once you approach a Galaxy Member, you will need to choose the “I want to see my Pokémon” conversation option, at which point you will be able to select which Pokémon you would like to have in your team (placing them in or removing them from the list that appears on the left-hand side of the screen).
When you are in Jubilife Village, you will need to speak to Marie who stands outside the Pastures facility on the eastern side of the bustling location. Her primary role is to help you when you feel like you have too many Pokémon stored in your Pastures, as you can choose to release your pocket monsters back into the wild. However, you can also use this menu screen to Move a Pokémon into or out of your party.