According to this page, renew the kernel first: http://wiki.openwrt.org/doc/howto/generic.sysupgrade
The page to download the latest image: http://downloads.openwrt.org/snapshots/trunk/ar71xx/
plug in the usb drive for large storage. According to this page, install packages to support USB:
http://wiki.openwrt.org/doc/howto/usb.storage
Use USB as memory:
http://wiki.openwrt.org/doc/howto/extroot
opkg update opkg install luci
go to 192.168.1.1 Network-> wifi, enable the network, attach lan to it, set security (mix), and the signal may look as 0%, try connect to the wiki once, then the signal will be correct.