Thanks a lot catymag and angelozip for your valuable instruction and timely help. I tried many many times with different fingure positions. Than I gave up and kept the unit in shutdown position for a long time. After that in one try it went into the pre-load mode and started loading. After the successful flushing of the rgn file, I successfully flushed the logo file also.
And now it is back to 3790. Thank you dears for your help.

I just need to know which files should I restore from the folder backup, so that the Garmin service desk will not know about this 3790-->3490 software change.