Generally, the userid and password to login to the router web server is on a label on the router. Usually, something like admin and admin. If you have changed it and can't remember, you have to do a factory reset. If the label is missing, the user manual for the router should have the userid and password.
If you mean the password to connect a wifi device, that is on the label also. If the label is missing, the user guide might indicate the default wifi password.
The admin password and wifi password are changed via the web server pages.