I found a good solution - delete the App on the Iphone and then reinstall it, and re-pair the device as new. Because the unsynced data is on the device and the rest is on the Garmin account, you don't loose any history.
My issue was my vivofit 3 stopped pairing after a few weeks of international travel. I had paired it overseas, and when I got home it wouldnt re-pair to change the time to local time. I deleted the app from the phone, and managed to re-pair it as a new device.
Worked!!