Dear DANIELE Thank you for contacting FOBO , Please release sensors before transferring to new user one by one by press and hold on sensor’s box and select "Release sensor" and then through App’s instruction to complete, Ensure sensors batteries are good. Should you have any issue arise, you may revert to us for further assistance by writing to fobo@salutica.com.my , Best Regards Amir Bayati FOBO Support Team