You can make a hard reset to your PulseOn device by pressing both buttons down for 20 seconds. After this press the right side button for 3 seconds to turn the device on. The device will turn on in "HR only mode".
More info on HR only mode and 3rd party apps: https://pulseon.freshdesk.com/support/solutions/articles/5000502802-how-to-use-pulseon-with-3rd-party-application-
To continue using PulseOn application again after a reset, you will need to undo old PulseOn pairing(s) from your mobile phone and pair again.
To set PulseOn device in pairing mode, please keep both buttons pressed down for 6 seconds, an icon will appear on the device screen.
Then, please pair your PulseOn again with the mobile application.