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You cannot change your original country setting if you have not linked to a Nintendo Account.
If you have already linked to a Nintendo Account, you can change your country setting in Animal Crossing: Pocket Camp by changing the setting in your Nintendo Account. From within your Nintendo Account, click on User info and then adjust your Country/region of residence. The setting will automatically be applied to your user profile in the game.
Note: Your country setting should only be changed if your country of residence changes. You must agree to the User Agreement and acknowledge having read the Nintendo Privacy Policy of the new country before being able to change the country setting.