Connect your computer to the router via Ethernet. Then log in to the router's IP address. The router is usually user/password protected. If you are not the one who normally manages the router, then ask whoever does.
Worse case scenario, you can hit and hold the reset button for a bit to reset the user/password to its default user/password. Be warned that if you do this, you lose all settings and changes and you will have to re-apply them. Which is why if you are not the person who normally manage it or you have no idea what you're doing, then I suggest you find help.