Double-tap the button on the right earbud, then on the third tap - press and hold the button for more than 5 seconds (until the unit shuts off). 
This restores the earbuds to the factory defaults.
TWS must be connected to perform the reset, so neither of the earbuds should be off or in the charging case.
It also may be necessary to remove or forget the earbuds from the list of paired Bluetooth devices in order to pair again.