Have a RB512 working just fine. I need to change configuration but cannot access it because during a trainning session some one as changed the admin password.

Can someone help with a step by step instructions on how to reinstall ?????

use netinstall

Thks Alex.

I know I have to use netinstall. The point is although I use Mikrotik Router Os for more than 2 years I never had the need to use netinstall.

I tried it but I run netinstall program and it does not detect any router ready to be reinstalled, so it does nothing. It is why I requested someone to help me with a step by step explanation. May be I am doing somethng wrong and I do not know what.

Thks once more and hope that some one can help me very soon.

simply running netisnstall will not work. you have to enable network booting on the board:

http://www.mikrotik.com/docs/ros/2.9/guide/netinstall