Ok guys, now I can confirm. Everything gone great. The navigation system is fully working now with fresh maps and polish interface. So I would like to say big, big thanks to You guys, especially Kunix and lolypop000. Without Your help I would have a brick.
For everybody with similar trouble. The procedure described by Kunix is only one right method, so in case of trouble with the unit's I suggest first ask our master;-)
FYI guys. In Poland Garmin service claim that the main board is damaged and irreparable. Only one method is change navi main board to brand new one. Cost about 300 euro. Drama.
Best regards for all.