You basically want to get to a boot prompt and boot single user mode with boot -s. Then execute a passwd command to reset the password and boot back normally. Here is some clue on how you may be able to do it on a NeXT box:

On Intel it gives you a boot prompt. Maybe it is different for next hw.
Is that what you are using? If so, I think the ns FAQ is still at
www.next.com.

I have something here that says type Command-Command-tilde on num-keypad
when the system says "testing system".