todo: explain upgrade procedure.

goto: www.openwrt.org
find the "bin"/firmware file corresponding to your device (if supported).

note:
There seems to always be TWO version of openWRT available: *Factory and *Sysupgrade.
The *Factory version is needed only when flashing from stock-manufacturer firmware TO openWRT.
*Factory needs to be special because stock-manufacturer have some checks built-in so that no wrong firmware is swallowed by the device and turns it into a brick. (it's a baby-DRM scheme?)
Once openWRT is installed, the *Sysupgrade version is then used to upgrade openWRT to newer versions of itself.

On the original web GUI of the device got to "firmware update".
select the downloaded file from openWRT (*Factory) and click update/brick button.
guess and wait for flashing to finish.

change the ip address of the computer to something in the 192.168.1.2-254 (netmask:255.255.255.0) range.
example: 192.168.1.2

it CANNOT be 192.168.1.1, which is the default ip-address of the newly flashed openWRT running device.

connect the computer via ethernet DIRECTLY to the openWRT device.

on computer, in a terminal / command-line, enter:
ssh root@192.168.1.1
press enter-keyboard at password prompt.

enter:
passwd
and enter new password for openWRT device. (maybe write it down?)

next configure ip-address, dns and gateway for openWRT device so it can be in the right place on the network.
example:
uci set network.lan.gateway=192.168.0.1
uci set network.lan.dns=8.8.8.8 (super-spy Google DNServer)
uci set network.lan.ipaddr=192.168.0.100

to make the above settings "stick" also enter:
uci commit

now it's time to reboot the device:
reboot
wait a few minutes for the device to reboot.

Next remove the device from ethernet cable (and power) and connect it in the right place on the network.
Also reset the ip-address of the computer to original ip and connect computer to original network.
-
to install a sane configuration interface in the year 2018+ do following:

connect to device again:
ssh root@192.168.0.100
enter the newly set password.

check setting for gateway and dns:
ping 192.168.0.1
ping 8.8.8.8

if both of above worked, shown by ping reply times, continue:
opkg update
opkg install luci

this will update repositories(?) and fetch a web-browser user-interface (webserver: uhttpd) to configure the device.

to run the web-config-interface, type:
/etc/init.d/uhttpd start and hit enter-keyboard.

the web-configuration interface should now be available via "http://192.168.0.100" in any browser that can connect to the openWRT device via ethernet.

to make the web-configuration interface stick around thru reboots, "enable" the interface via:
/etc/init.d/uhttpd enable

link: https://openwrt.org/docs/guide-quick-start/webadmingui
link: https://openwrt.org/faq/before_installation
link: https://openwrt.org/faq/after_installation
link: https://wiki.openwrt.org/doc/uci