(BTW, many are about to tell you that the device is old and that the firmware should not be ran in a production environment due to the many security vulnerabilities in the software - discovered over the years.)
Example - flash to 10.03.1:
cd /tmp/
wget http://downloads.openwrt.org/backfire/10.03.1/brcm47xx/openwrt-brcm47xx-squashfs.trx # The file must be the trx file.
mtd write /tmp/openwrt-brcm47xx-squashfs.trx linux && reboot