Step 1: Into the onie_rescue first.
When you reboot the switch, you will see this message like below the picture.
Please type any key, then you will get into the uboot.
After that, please type "run onie_rescue", then you will get into the ONIE.
Step 2: Setting the IP address for management port.
Step 3: update the ONIE.
By the way, you could use tftp for updating, too.