From OpenFlow Wiki

Flash Process

The Bullet can be flashed like most of the Ubiquity devices in the recovery mode via TFTP.

• Connect your computer and the Bullet with a network cable
• Give your computer a IP in the 192.168.1.0/24 network except the 192.168.1.20
• Boot the Bullet in recovery mode by holding the reset switch for a few seconds while powering on the device.
• Upload the new firmware to the device using TFTP to the device on the default IP

hostname >>tftp 192.168.1.20
tftp> binary
tftp> trace
Packet tracing on.
tftp> put openwrt-atheros-ubnt2-squashfs.bin

address, 192.168.1.20.

• Wait at least 7 minutes, before you unplug the power from the Bullet.