The website says that there is a firmware update for the Garmin Edge 500. But when I click on the suggested link from Garmin's to do the update, it mentions all the models with the available update. Like the Forerunner line. But the Garmin Edge 500 is not in the list.
The easiest way to update an Edge device is to download the Garmin WebUpdater, which once your device is connected, will automatically check and pull any applicable firmware updates to your device. I'd be surprised if the 500 is considered current anymore. Once a device isn't current, they stop updating the firmware and that's usually just shortly after the successor is announced.
It's also possible to reinstall previous versions, but not through any Garmin facility. You need the appropriate file and to use the appropriate procedure. Some find themselves doing this as an update sometimes screws up some vital function that was working in earlier version. Personally, I'm running a down rev on all my Garmin Edge units.