Once a Nintendo Account has been linked to Animal Crossing: Pocket Camp, the link cannot be removed or changed. Also, unless you delete your Animal Crossing: Pocket Camp save data, you cannot switch to another Nintendo Account.
To delete your save data, you can follow these steps.
From within the game:
❶ Select More.
❷ Select Settings.
❸ Select Nintendo Account Management.
❹ Select Delete All Save Data.
❺ Delete your save data.
Please note that your data (including any Leaf Tickets you may have purchased or obtained) cannot be restored once deleted.