You can access your router's configuration page through a web browser on a computer connected to your network. If you can't connect via Wi-Fi because you don't know the password, use an Ethernet cable to connect your computer directly to the router. This will bypass the need for the Wi-Fi password.
Standard router addresses are 192.168.1.1, 192.168.0.1, 192.168.2.1, or 10.0.1.1 (Apple) or 10.0.0.1 (Xfinity). Enter the address into your browser's address bar. If none of the above addresses grant you access to the configuration page, open the Command Prompt by pressing the Windows key + R and entering cmd. Once the Command Prompt opens, type ipconfig and press Enter. Look for your active connection in the list and find the Default Gateway address. This is typically your router address. If all else fails, press and hold the Reset button on your router for about 30 seconds to revert it to factory default settings. Then, look up the default address for that router model and enter it into your browser. Some routers come with configuration software. If you've previously installed your configuration software, you can use that instead of the web browser interface.