Download the bin file, go to the nokia 770 website as the docs say, find a lack of downloads, then go to the 810 page, download the flashing program, run it (assuming a windows machine here), connect a USB lead, restart the 770 holding the home key, and follow the instructions. Shortly after, you have update.

Did it this afternoon. The webbrowser seems flakey as anything, and I no longer can see my wireless (saw it once to add it to the list, now it can't find anything). Other than that it works ok!