Either
- You have the router password: Browse to 192.168.0.1, enter it, and change the settings.
- You have physical access to the router. Unplug it, press and hold the reset button, plug it back in. wait some 30 seconds, then let go of the reset button. The router will have lost all settings. Browse to 192.168.0.1, and set it up newly. Note that you might get in non-technical-related trouble later...