The old Administrator should go to the Angel Watch app, go to Family members and delete all. Then tap ME at the bottom, then Device List, click Edit, and delete the watch. This will unregister the Administrator and all other bound accounts.
The new Administrator should delete the device from their Device List (if added) and can then rebind the watch to become the new Administrator.
For more assistance, please reach out to us at firstname.lastname@example.org or by calling us at 1-888-787-4757 or 307-336-7051.