the easiest way is to plug directly into the router using an ethernet cable.
Open up internet explorer and type in 192.168.1.1 this (usually) will take you into the interface for the router, where you can change or reset the password.
You will have to enter a username and pass to get into the router settings page but most likely it has a default username and password.
Usually those names work but you may have to do some research online to get the pw names.
Or...... You could just unplug it and hold in the reset button to restore it to the day you took it out of the box.