Recently I installed OpenWRT on Ubiquiti AirRouter as original firmware lacks in couple of areas. Installation is easy and straightforward as all you need to do at the time of writing this post is to use Attitude Adjustment release.
USB storage can be automatically mounted and shared over network which is main reason I switched to OpenWRT.
For the first time get openwrt-ar71xx-generic-ubnt-airrouter-squashfs-factory.bin image. To download it browse to ar71xx/generic/ sub directory using Attitude Adjustment release link. Alternatively you can use latest snapshot directory.
Access AirRouter using browser, navigate to the System tab and upload downloaded image.
Afterwards you need to confirm your decision.
Router will reboot after upgrade.
To later upgrade OpenWRT to the latest available version use openwrt-ar71xx-generic-ubnt-airrouter-squashfs-sysupgrade.bin image.
If you need to revert back to AirOS then at first search OpenWRT wiki for similar devices. Procedure is very simple and can be repeated if something goes wrong.
1. Download AirOS firmware (for example I will use XM-v5.5.2.build14175.bin).
2. Turn off device.
3. Connect directly to the device using Ethernet cable.
4. Set the computer IP address to 192.168.1.11/24. IP address can be different (using the same subnet) but not 192.168.1.20.
5. Push the reset button, turn on device and keep reset button pushed for around 10 seconds.
6. Ping IP address 192.168.1.20 - if it doesn't respond then turn off device and start again.
7. Put AirOS using tftp:
$ tftp 192.168.1.20 tftp> mode binary tftp> put XM-v5.5.2.build14175.bin flash_update Sent 6736122 bytes in 9.4 seconds tftp> quit
8. Watch leds for change and then wait couple of minutes just to be safe.
9. Reboot device by hand.