The most sure-fire way to do this is to start a command-line session with the router, erase the startup-config and reload the router. The command syntax for that would be:
*If you don’t have the login credentials for the router to get in, do this:
-Start a terminal session with the router via the Console port (telnet will not work for this). Physically reload the router, and from the console repeatedly hit the “Break” key, normally on the upper right hand side of the keyboard.
-You should now be in ROMMON mode. Type “confreg 0x2142” without quotes and hit Enter. Now type “reset” without quotes and hit Enter.
-Allow the router to restart, and hit “n” followed by Enter to stop the startup wizard.
-Type the following command sequence:
The router will now boot normally, with a factory default configuration, even if you did not know any of the password(s) securing the router.