As usual with this scooter, just getting access to the bulb is the hard part. The bulb is in a protective housing just over the license plate (duh!).
First, remove the license plate, which is held on by a bolt on either side of the plate. You'll then be looking at the bracket.
Remove the two nuts in the center of the bracket.
You can now remove the bracket by carefully lifting it off the bolts. There are rubber gaskets that tend to grip the bolts, so it's a little tricky. Be careful not to tug on the wires that go between the body of the scoot and the light housing; the leads are pretty short.
After removing the bracket, turn it around so you can access the "special screws" (as the maintenance manual calls them) that hold the light housing together.
Remove the special screws and disassemble the housing. Here are the parts that come off.
Reverse the steps to reassemble: Put the housing back on with the special screws, replace the bracket on the bolts, replace the nuts, then remount the license plate.
Thanks to Steve for suggesting the part.