Before you do this beware that flashing the firmware may invalidate your warranty or stop your device from working correctly. You do so at your own risk. The basic procedure for uploading/flashing a firmware image is described here for the Level one WAP-0007 - it is much the same for other devices.

The basic info (first two paragraphs below) comes from http://forum.openwrt.org/viewtopic.php?pid=35617#p35617

First of all it’s important to upgrade to Version R400b7, or you won’t be able to flash your selfmade image to your NAS. Not all vendors provide such an image, however the conceptronic one works fine. You can get it from http://download.conceptronic.net/CONNECTIVITY/CHD2WLANU/CHD2WLANU_FW_R400b7.zip. Note that the printer server feature does not currently work either with this image or with a modified one.

Then you need an Hexeditor to copy the header from your old firmware file over the header from the new (R400b7) firmware file. I took the whole stuff from the first byte until three bytes before “NONE”. If you look at your binary you know what I mean.

It varies slightly from brand to brand as to exactly which firmware you need to update the header from. For the Safecom SWSAPUR-5400, I used the conceptronic firmware b7 with the header from b4 as used by biro in http://forum.openwrt.org/viewtopic.php?pid=39433#p39433. I did not find it necessary to “downgrade” to the conceptronic b4 first. In other words I first flashed with the image from http://www.penn8688.freeserve.co.uk/temp/CHD2WLANU_R400b7unlock.BIN, then the next flash can be a modified image. This procedure may well work for other brands as well.