In order to get your router back to the original settings, you must push in the reset button (usually on the back of the router) for somewhere between 10 and 30 seconds depending on the router. You may need a pin or something small to push in the button, because it can be recessed. Once you have reset the router, the username and password will be changed to the defaults, admin and password, respectively.
One thing you must remember is that resetting the router will change all of the router settings back to the factory settings. This means you will lose all of the changes you made since you got the router. Make sure to write down any things you’ve changed, so you can change them back after the reset. Also, I recommend updating to the latest firmware after the reset.