There is no simple way to backup firmware but it is a good idea to backup all content of your nuvi to PC. Connect nuvi to PC by USB cable, it will be recognized as removable drive, and simply copy all files to PC.
All firmware versions for nuvi can be downloaded from Garmin, but there is no searchable directory on Garmin site. You can search for upgrade info like this one:
You will find a link to firmware if you look at html source for this page :)
Some firmware versions can be found here:
Usually you can flash older firmware version without problems, but there are some cases, when flashing is not possible or can damage gps.