Returning D-Link DIR-505L to factory firmware

[mehdi]

I've been successfully using a D-Link DIR-505L for Meteobridge for a few years, but recently changed to a Raspberry Pi.
Now I would like to restore the D-Link factory firmware on the DIR-505L before selling/giving it up.

I've already downloaded the latest (very old) firmware from the D-Link site, but how do I go about uploading it to the router? I've tried uploading the D-Link FW via "http://ip.of.my.device/cgi-bin/luci/" and "http://ip.of.my.device/cgi-bin/fwupdate.cgi" without any success. It seems the factory FW is not in an acceptable format.

What is the correct way to reload the factory FW on the DIR-505L?

Thanks.