The data your device has tracked is associated with your Bellabeat account within the app. That is why even if you change phones, once you log into your account all your previously recorded data will be available.
If for some reason you wish to start fresh and not have the old data visible in the account, you will just need to create a new account in the Bellabeat app with a different email and the device will seem to you as a brand new one, without any previous information visible.