There's a lot of software on the web for the various GPS models designed to download tracks, routes etc from your unit to a PC (and vice versa). I've just come across a software package at
http://www.abnormal.com/~avia_dbm/ which goea a bit further and allows you to alter the aviation database for some Garmin models. The only drawback is that you must already have bought a Garmin database upgrade as the program can only access data on the disks (it can't grab the database in the unit).
It appears to work fine - I've added 75 UK airfields missing from the Garmin unit and stuck the new runway in at Manchester!