This serves two purposes. First, it gets your 770 update (in Nokia's eyes), and second, if you messed something, this will probably get it back to normal.
Note: if you have anything on your Maemo, you'll want to back it up first.
Follow the directions here.
Mac users have two options, according to the maemo wiki. However, I recommend following Nokia's advice just because it's not that much more difficult.
On your computer, download the latest flasher from Nokia.
For me, on Mac OS X (PPC), I downloaded "flasher-2.0.macosx"
Go to Nokia to download the latest image (.bin) file. It will tell you to turn your 770 off, remove the battery, and type in your WLAN number to gain access.
For me, this was "SU-18_2006SE_3.2006.49-2_PR_F5_MR0_ARM.bin", a 60.1 MB file.
Open the terminal (on the Mac, Applications/Utilities) and cd to the directory where the flasher/bin files. If they are on the desktop, you'd type
cd ~/Desktop
and then
chmod +x ./flasher.macosx
This gives the flasher binary execute permissions (x).
Remove the MMC card from the 770, unplug the power cable, and turn it off. Connect it to your computer via USB.
In the terminal, type
sudo ./flasher-2.0.macosx -F SU-18_2006SE_3.2006.49-2_PR_F5_MR0_ARM.bin -f -R
You should get a message that says, "Suitable USB device not found, waiting".
With the USB cable plugged in, hold down the home key, and press the power button until the Nokia screen turns on. You should see the USB triton appear in the upper-right corner of the screen. The terminal should say, "Found device...". When it is finished you'll see "Finishing flashing... done".
Unplug the USB cable and power the device on.
You now have a clean system.