zikmir, you were directed to this thread from this other thread http://talk.maemo.org/showthread.php?t=35998 so you could know how to recover your lock code without having to reflash your device. Then, why are you still asking how to reflash it?

So if you are still interested in recovering your lock code without reflashing, and only if you had installed an openssh server in your N900 so you can remotely access it from your PC, then please read comment #5 of this thread, get your encoded code, transfer it to your PC (scp) and feed it to a DES decoder. A Google search will show you a plenty of linux/windows/mac DES decoders.

Otherwise, if you insist reflasing your device read here or you don't have openssh installed in your N900, presumably then look at this:

http://wiki.maemo.org/Updating_the_tablet_firmware (though you should have been able to find this info by yourself searching the forums, as this has been posted several times)

Good luck.