If you're currently using a Fitbit with Walkadoo but want to start using a different Fitbit device instead, there's actually nothing you'll need to change in Walkadoo, since Walkadoo is already linked to your Fitbit account.
Instead, you'll need to update your preferred Fitbit device in your Fitbit.com account, which you can learn more about here. Please contact the Fitbit support team if you need additional assistance making this change.
Once your preferred Fitbit device is syncing to your Fitbit.com account, you'll see updated steps in Walkadoo within about an hour of syncing your Fitbit. You can find a bit more information on this topic here.