Just go to the website your key already registered. On the 2-step verification tab or similar tab, delete the device. Two FIDO keys are recommended, one for normal use, the other for backup.
iPhone does not allow the FIDO Key to be paired over the Bluetooth setting page directly. You need to download the application “Smart Lock” from app store and use the “Smart Lock” to pair the FIDO Key.
Most of the FIDO® U2F PC applications rely on HID interface which Bluetooth is not compatible with that. Please use the FIDO key on PC over USB interface.