Once a box already has openwrt or cerowrt on it, you can reflash via the web based utility. Generally, to be safe, it’s a good idea to use the command line tool.
The -sysupgrade.bin image is always used when upgrading OpenWrt (CeroWrt) images, e.g. openwrt-ar71xx-generic-wndr3700v2-jffs2-sysupgrade.bin.
cd /tmp wget the_image_file (located at http://huchra.bufferbloat.net/~cero1/_the release candidate_/ sysupgrade -d 60 -n the_image_file
Enough is in flux in CeroWrt for it to be a good idea to always reflash from scratch, and then selectively install your customizations from a backup of /etc.
You do have a backup, don’t you?