Installation using a USB Thumb Drive
This section explains how to transfer the image from an USB thumb drive into the device and install it.
Copy the downloaded SONiC image on the USB thumb drive.
Remove the USB drive from your machine and insert it into the USB port on the front (or rear) panel of your ONIE enabled device.
Power on the device and ONIE will discover the onie-installer file on the root of the USB drive and execute it.
The above information is taken from https://opencomputeproject.github.io/onie/user-guide/index.html