This is going to be a quick reference vlog on how to put the Particle device (Photon, Boron, Electron) into a DFU mode (Device Firmware Upgrade mode), which technically allow you to load your firmware via USB.
When the Particle device is in DFU mode, the LED will flash yellow.
In the nutshell, this is how to enter DFU Mode.
1) Apply power to the board
2) Hold down both buttons (MODE & RESET)
3) Release the RESET button, while holding down MODE button
4) Release the MODE button when yellow LED starts flashing (after magenta)
I hope it helps you.