You can do a factory reset on your V650. It removes the V650’s connection to your user account, and deletes any saved data on it.
Plug in your V650 to your computer’s USB port and open the Polar FlowSync software. The Polar FlowSync software transfers your training information to the Polar Flow web service.
Go to Settings and choose Factory Reset.
If you want to continue using Polar V650, remove the USB cable and plug it in again. After this, the Polar FlowSync software asks you to set up the device in the Polar Flow web service (https://flow.polar.com).