Where is this thumb print recorded,, there is no thumb print with the Flex.
Apple has touch ID is that what your reffering to?
I've moved your post to the iOS area. Please follow the link for instructions on setting up the touch iD, it looks like you just need to set it up with your thumb.
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@Katetin I believe this is an iOS device settings, not a Fitbit settings. You should be able to edit it by:
1. Opening your iOS settings.
2. Click on Touch ID & Passcode.
3. Plug in your iOS pin (if you have one).
4. Adjust your Touch ID settings.
Hope this helps you out!