Hi Joerg, I use an older Garmin device, so have not tested what follows: there's this nice blog in German: https://blog.dafb-o.de/neue-firmware-fuer-garmin-devices-unter-linux/ where the firmware update procedure from Linux is detailed and a link to the firmware files for your device is given http://gawisp.com/perry/edge/ .
Have fun, jvp.
