Thank you very much Catymag for that link! It took me several tries, but it worked. These are the steps that worked with my Nuvi 55LM. With the WebUpdater running on the PC loaded with the .rgn file and USB cable attached to the back of the Nuvi:
1. Press and hold the power button and plug in the USB cable to PC simultaneously
2. When the Garmin logo appears, press and hold the top left corner of the screen on the Nuvi. Click 'OK' in the Updater window for "USB Device" when it appears on the dropdown box
3. When updater begins to flash the firmware, release the power button on the Nuvi, but continue depressing the top left corner of the screen.
4. Release your finger from the Nuvi screen after updater displays a "successful" message (about 60 seconds after start)