To reset your Fitbit Flex, plug your charging cable into the USB port, and then insert the tracker into the charging cable. Next, insert a paperclip into the small pin-sized hole on the back of the charger, and hold it there for three or four seconds.
After holding the pin in the hole for three or four seconds, remove it. Your Fitbit Flex has been restarted, and you can unplug it from the charger and reinsert it into the wristband.
If your tracker was not syncing or responding to movement, wasn't responding when it was in a charger, charged but was not lighting up, or was experiencing similar problems, those problems should be resolved by resetting or restarting your tracker.
Restarting your Fitbit Flex does not clear its tracking data. If you want to reset your FitBit Flex's data, log into your Fitbit.com dashboard, click Log and then the Activity tab underneath it, and select the data you want to remove. FitBit does not allow you to remove data, unless you sync it to a different account altogether, but you can accomplish the same goal by negating the data so it does not count. Type "Driving" in the activity menu and enter the duration and start time, then click the log activity where you see the steps, floors and calories decrease for the specified time.