first configure your local computer to 192.168.1.23
then, install a ftp server (not tftp)
place firmware.img into the root folder of this ftp server

now go to 192.168.1.250 using your webbrowser and login with empty username and password admin
use the firmware upgrade tab and use preflash.img as firmware upgrade file

after the unit rebooted. go again to 192.168.1.250 with your webbrowser
and visit http://192.168.1.250/upldadv.htm


type in the following data in the provided fields

hostname : 192.168.1.23
username: anonymous
password:
Image Path: /
Image Name: 2nd.img

then press update firmware

consider that now just a "go back" button will be display. press it, then you will be redirected to the same page again
now press "update firmware" again

after the firmware has been flashed, press the "reboot ap" button which will be displayed