Thanks for your reply. I tried doing this from the thread you had linked:

Pre-Boot for navigators nuvi 1200 1210 1300 1310 1350 1390 1410 1490:

1. Turn off the navigator.

2. Press the upper left corner of the screen.

3. Turn on the navigator still holding the upper left corner of the screen

I held the upper left corner for over 5 minutes but still nothing on the screen execept for "Garmin"

Is there another way of getting into preboot mode?