There are several ways the upgrade the router. The flashing itself only takes 5 min (instead of 20 min with FON2202).
There are two image flavors :
If you did not configured the fonera through the web interface, or skipped it, the default password of the web interface and SSH is admin.
the easiest is to use the web interface (http://192.168.10.1 or http://fonera). Log in and you will arrive on the dashboard, go into in the applications. If there is a firmware upgrade, it will show a section. You then just need to press on “+”
If you have the developer image, you can access the fonera over SSH.
cd /tmp wget -O fonera.img http://download.fonosfera.org/Gari/20100408_FON2303_22.214.171.124_r1665_DEV.img
mtd -r write fonera.img image
source : fon wiki
FON2303 use the bootloader u-boot (FON2202 uses RedBoot)
If the fonera stopped working, is not accessible through the web interface or SSH, then there is still the restore option.
wget -O fonera.bin http://download.fonosfera.org/Restore/20091008_FON2303_126.96.36.199_Restore_DEV.bin
sudo aptitude install tftp-hpa
sudo ifconfig eth0 192.168.1.1
tftp -v 192.168.1.6 -m binary -c put fonera.bin